arm64: gic: Enable access to ICC_* registers

Thi GICv3 driver is configuring the controller accessing the system
registers ICC_*. To be able to do that without trapping we have to
explicitly set at boot in EL3 the value of the ICC_SRE_EL3 register that
is architecturally set to UNKNOWN value on warm reset.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>