csky: Add memory layout 2.5G(user):1.5G(kernel)

There are two ways for translating va to pa for csky:
- Use TLB(Translate Lookup Buffer) and PTW (Page Table Walk)
- Use SSEG0/1 (Simple Segment Mapping)

We use tlb mapping 0-2G and 3G-4G virtual address area and SSEG0/1
are for 2G-2.5G and 2.5G-3G translation. We could disable SSEG0
to use 2G-2.5G as TLB user mapping.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

csky: Separate fixaddr_init from highmem

After fixaddr_init is separated from highmem, we could use tcm
without highmem selected. (610 (abiv1) don't support highmem,
but it could use tcm now.)

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>

csky: Tightly-Coupled Memory or Sram support

The implementation are not only used by TCM but also used by sram on
SOC bus. It follow existed linux tcm software interface, so that old
tcm application codes could be re-used directly.

Software interface list in asm/tcm.h:
- Variables/Const: __tcmdata, __tcmconst
- Functions: __tcmfunc, __tcmlocalfunc
- Malloc/Free: tcm_alloc, tcm_free

In linux menuconfig:
- Choose a TCM contain instrctions + data or separated in ITCM/DTCM.
- Determine TCM_BASE (DTCM_BASE) in phyiscal address.
- Determine size of TCM or ITCM(DTCM) in page counts.

Here is hello tcm example from Documentation/arm/tcm.rst which could
be directly used:

/* Uninitialized data */
static u32 __tcmdata tcmvar;
/* Initialized data */
static u32 __tcmdata tcmassigned = 0x2BADBABEU;
/* Constant */
static const u32 __tcmconst tcmconst = 0xCAFEBABEU;

static void __tcmlocalfunc tcm_to_tcm(void)
{
int i;
for (i = 0; i < 100; i++)
tcmvar ++;
}

static void __tcmfunc hello_tcm(void)
{
/* Some abstract code that runs in ITCM */
int i;
for (i = 0; i < 100; i++) {
tcmvar ++;
}
tcm_to_tcm();
}

static void __init test_tcm(void)
{
u32 *tcmem;
int i;

hello_tcm();
printk("Hello TCM executed from ITCM RAM\n");

printk("TCM variable from testrun: %u @ %p\n", tcmvar, &tcmvar);
tcmvar = 0xDEADBEEFU;
printk("TCM variable: 0x%x @ %p\n", tcmvar, &tcmvar);

printk("TCM assigned variable: 0x%x @ %p\n", tcmassigned, &tcmassigned);

printk("TCM constant: 0x%x @ %p\n", tcmconst, &tcmconst);

/* Allocate some TCM memory from the pool */
tcmem = tcm_alloc(20);
if (tcmem) {
printk("TCM Allocated 20 bytes of TCM @ %p\n", tcmem);
tcmem[0] = 0xDEADBEEFU;
tcmem[1] = 0x2BADBABEU;
tcmem[2] = 0xCAFEBABEU;
tcmem[3] = 0xDEADBEEFU;
tcmem[4] = 0x2BADBABEU;
for (i = 0; i < 5; i++)
printk("TCM tcmem[%d] = %08x\n", i, tcmem[i]);
tcm_free(tcmem, 20);
}
}

TODO:
- Separate fixup mapping from highmem
- Support abiv1

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>