ARM926EJS: Make asm routines volatile in cache ops

We certainly don't want the compiler to reorganise the code for dcache flushing.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Stefano Babic <sbabic@denx.de>