xtensa: add XIP-aware MTD support

Add Kconfig symbol, asm/mtd-xip.h header and define related macros:
xip_cpu_idle, xip_irqpending, xip_currtime and xip_elapsed_since.
Append .xiptext to the vectors when separate vectors are configured
because otherwise they are appended to the data and data may not even be
executable in XIP configuration.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

xtensa: fix declaration of _SecondaryResetVector_text_*

Secondary reset vector is defined, compiled and used when
CONFIG_SECONDARY_RESET_VECTOR is enabled, not only on SMP.
Make declarations of _SecondaryResetVector_text_* symbols available
accordingly.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

xtensa: move section symbols to asm/sections.h

Introduce asm/sections.h and move section declarations to this header
from setup.c. Assign section symbols char array type uniformly and drop
address operator from section symbol references in code.
Sort headers in setup.c while at it.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>