FVP: Fix AArch32 stack functions to be ABI-compliant

FVP: Fix AArch32 stack functions to be ABI-compliant

plat_get_my_stack is called from C, so it can't expect argument
registers to be preserved. Stash registers temporarily onto the stack
instead.

plat_set_my_stack is called during early init, when there exists no
stack. Use any register other than argument registers to stash temporary
values.

Change-Id: I98052e20671d0933201d45ec7a5affccd71ce08c
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>