SPL: Allow ARM926EJS to avoid compiling in the CPU support code

This allows the SPL to avoid compiling in the CPU support code.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Scott Wood <scottwood@freescale.com>