arm, arm926ejs: make thumb mode compileable

arm, arm926ejs: make thumb mode compileable

in thumb mode compiler says for example for arch/arm/lib/cache-cp15.c
when enabling CONFIG_SYS_THUMB_BUILD:

{standard input}: Assembler messages:
{standard input}:373: Error: selected processor does not support Thumb mode `mrc p15,0,r4,c1,c0,0'
{standard input}:416: Error: selected processor does not support Thumb mode `mcr p15,0,r3,c2,c0,0'

so, if caches are disabled, do not use this command on arm926ejs.
used on at91 in SPL, to reduce size of SPL.

Signed-off-by: Heiko Schocher <hs@denx.de>