fix(el3-runtime): set unset pstate bits to default

During a transition to a higher EL some of the PSTATE bits are not set
by hardware, this means that their state may be leaked from lower ELs.
This patch sets those bits to a default value upon entry to EL3.

This patch was tested using a debugger to check the PSTATE values
are correctly set. As well as adding a test in the next patch to
ensure the PSTATE in lower ELs is still maintained after this change.

Change-Id: Ie546acbca7b9aa3c86bd68185edded91b2a64ae5
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>