aarch32: Always enter kernel via exception return

When the boot-wrapper is entered at Secure PL1 it will enter the kernel
via an exception return, and when entered at Hyp it will branch to the
kernel directly. This is an artifact of the way the boot-wrapper was
originally written in assembly, and it would be preferable to always
enter the kernel via an exception return so that PSTATE is always
initialized to a known-good value.

Rework jump_kernel() to always enter the kernel via an exception return,
matching the style of the AArch64 version of jump_kernel()

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Cc: Akos Denke <akos.denke@arm.com>
Cc: Luca Fancellu <luca.fancellu@arm.com>