bl2-el3: Fix exit to bl32 by ensuring a more complete write to SPSR

bl2-el3: Fix exit to bl32 by ensuring a more complete write to SPSR

Prior to entry into BL32 we set the SPSR by way of msr spsr, r1.
This unfortunately only writes the bits f->[31:24] and c->[7:0].

This patch updates the bl2 exit path to write the x->[15:8] and c->[7:0]
fields of the SPSR. For the purposes of initial setup of the SPSR the x and
c fields should be sufficient and importantly will capture the necessary
lower-order control bits that f:c alone do not.

This is important to do to ensure the SPSR is set to the mode the platform
intends prior to performing an eret.

Fixes: b1d27b484f41 ("bl2-el3: Add BL2_EL3 image")
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>