xtensa: fix ATOMCTL register documentation

Make the WT entry match table 4-52 of the Xtensa ISA RM (RD-2012.5).

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: save current register frame in fast_syscall_spill_registers_fixup

Also fix comments and function bounds annotations.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: fix a2 damage in fast_syscall_spill_registers_fixup

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