ARM: cache: add an empty stub function for invalidate/flush dcache

Since some driver like ohci, lcd used dcache functions. But some ARM
cpu don't implement the invalidate_dcache_range()/flush_dcache_range()
functions.

To avoid compiling errors this patch adds an weak empty stub function
for all ARM cpu in arch/arm/lib/cache.c.
And ARM cpu still can implemnt its own cache functions on the cpu folder.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: York Sun <yorksun@freescale.com>