Arm's GIC-600 features a Power Register (GICR_PWRR),
which needs to be programmed to enable redistributor
operation. Section 3.6.1 in the GIC-600 TRM describes
the power-up and power-down sequence in pseudo code,
which deviates from the current TF-A implementation
in drivers/arm/gic/v3/gic600.c.
For powering on a redistributor, the pseudo code suggests
to loop over the whole sequence (check for transition,
write request bit) instead of just looping over the
ready bit read as TF-A does in gic600_pwr_on().
This patch fixes GIC-600 power up sequence according
to the TRM.

Change-Id: I445c480e96ba356b69a2d8e5308ffe6c0a97f45b
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>