armv8/cache: Change cache invalidate and flush function

When SoC first boots up, we should invalidate the cache but not flush it.
We can use the same function for invalid and flush mostly, with a wrapper.

Invalidating large cache can ben slow on emulator, so we postpone doing
so until I-cache is enabled, and before enabling D-cache.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: David Feng <fenghua@phytium.com.cn>