ARM: GIC: Convert GIC library to use the IO relaxed operations

ARM: GIC: Convert GIC library to use the IO relaxed operations

The GIC register accesses today make use of readl()/writel()
which prove to be very expensive when used along with mandatory
barriers. This mandatory barriers also introduces an un-necessary
and expensive l2x0_sync() operation. On Cortex-A9 MP cores, GIC
IO accesses from CPU are direct and doesn't go through L2X0 write
buffer.

A DSB before writel_relaxed() in gic_raise_softirq() is added to be
compliant with the Barrier Litmus document - the mailbox scenario.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>