armv7: add PL310 support to u-boot

PL310 is the L2$ controller from ARM used in many SoCs
including the Cortex-A9 based OMAP4430

Add support for some of the key PL310 operations
	- Invalidate all
	- Invalidate range
	- Flush(clean & invalidate) all
	- Flush range

Signed-off-by: Aneesh V <aneesh@ti.com>