[ARM] armv7: add dedicated ARMv7 barrier instructions

Starting with ARMv7, there are dedicated instruction for the ISB, DSB
and DMB barriers and there is no need to execute them as CP15
operations.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>