BL31: Enable pointer authentication support in warm boot path

In the current Pointer Authentication support added in
commit b86048c4
PAuth gets enabled in BL31 cold boot entrypoint only,
(see bl31_entrypoint() in bl31\aarch64\bl31_entrypoint.S)
but not in bl31_warm_entrypoint().
This results in EnIA bit [31] in SCTLR_EL3 not being set
and pointer authentication disabled after CPU wake-up event.

Fixes ARM-software/tf-issues#684

Change-Id: I27a67804764dfba2a6d72ca119ca2bcff4f536d6
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>