xtensa: handle memmap kernel option

This option is useful for reserving memory regions for secondary cores
in AMP configurations.

Implement the following memmap variants:
- memmap=nn[KMG]@ss[KMG]: force usage of a specific region of memory;
- memmap=nn[KMG]$ss[KMG]: mark specified memory as reserved;
- memmap=nn[KMG]: set end of memory.

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

diff --cc arch/xtensa/kernel/setup.c
Simple merge

diff --cc arch/xtensa/mm/init.c
Simple merge