ARM: l2c: provide common PL310 early resume code

Provide a common assembly implementation for PL310 resume code.  Certain
platforms need to re-initialise the L2C cache early as it may preserve
data across a S2RAM cycle, and therefore must be enabled along with the
L1 cache and MMU.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>