Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

Move some lockTLB magic into C.

This patch moves the 'outer' chunk of lockTLBEntry into C rather
than handwritten assembly. The outer chunk accesses a global
counter and does arithmetic. The inner chunk (lockTLBEntryCritical)
writes to the registers, must be specially aligned, and is generally
special.

The change reduces unnecessary handwritten assembly, and also avoids
a special case that was problematic for binary verification.

Use CONFIG_ variants of configuration definitions

The use of the CONFIG_ variants (that get declared in autoconf.h) for configuration
variables is preferred over definitions that get set somewhere in some Makefile

SELFOUR-723: kernel config for the hikey platform

Also:
- cleanup unused Cortex A57
- Cortex A53 is v8 so removing 'ARM_CORTEX_A53' form v7 files
- define rpi3 as v8

SELFOUR-537: Support for raspberry pi3

Make lockTLBEntry comply with the decompiler

lockTLBEntry, an assembly function, had tlb_lock_count as a symbol that
needed to be placed sufficiently close to be loaded and stored with
offset-from-pc addressing. When assembled, the symbol would turn up
between functions, as opposed to within a literal pool (it's a variable,
not a constant) or the .bss / data sections. The decompiler doesn't
handle that use case, and likely won't. This change turns
tlb_lock_count into a C global variable (so that it will be placed in
the .bss / data sections), and splits lockTLBEntry into two parts so the
critical section will still fit in a 64-byte aligned region, and
therefore be guaranteed to live within a single page.

arm/imx7: add support for iMX7-based Sabre board

Release snapshot