msm: 8x60: gic initialization fixup for RUMI

On RUMI platform STIs are not enabled by default, contrary to the
GIC spec. The bits for STIs in the enable/enable clear registers
are also RW instead of RO. STIs need to be enabled at initialization
time.

Signed-off-by: Steve Muckle <smuckle@codeaurora.org>
Signed-off-by: Daniel Walker <dwalker@codeaurora.org>