ARM: GIC: function to retrieve the physical address of the SGIR

ARM: GIC: function to retrieve the physical address of the SGIR

In order to have early assembly code signal other CPUs in the system,
we need to get the physical address for the SGIR register used to
send IPIs.  Because the register will be used with a precomputed CPU
interface ID number, there is no need for any locking in the assembly
code where this register is written to.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
(cherry picked from commit eeb446581ba23a5a36b4f5c7bfa2b1f8f7c9fb66)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>