armv7: import new cache maintenance API from coreboot

This imports the new cache maintenance API from coreboot at
commit bba8090. This is a BSD-licensed implementation which
exposes cache maintenance opertaions necessary for payloads
for things such as DMA transfers.

Change-Id: I554676db89517bebc6edae4f7ab7e5882e6f986d
Signed-off-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/2974
Tested-by: build bot (Jenkins)

payloads/libpayload/arch/armv7/Makefile.inc

@@ -33,4 +33,4 @@ libc-y += timer.c coreboot.c util.S
 libc-y += virtual.c
 libc-y += memcpy.S memset.S
 libc-y += exception_asm.S exception.c
-
+libc-y += cache.c