Fix AArch32 build error

Pointer authentication code should be conditionally compiled only if
TFTF is built for AArch64 since AArch32 doesn't support pointer
authentication. This was previously causing a compilation error.

Change-Id: I97a0ea66f562cf2ebb41478b1b31a26bcaf815b2
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>