sunxi: armv8: FEL: save and restore SP_IRQ

sunxi: armv8: FEL: save and restore SP_IRQ

Thanks for Jernej's JTAG debugging effort, it turns out that the BROM
expects SP_IRQ to be saved and restored, when we want to enter back into
FEL after the SPL's AArch64 stint.
Save and restore SP_IRQ as part of the FEL state handling. The banked
MRS/MSR access to SP_IRQ, without actually being in IRQ mode, was
introduced with the ARMv7 virtualisation extensions. The Arm Cortex-A8
cores used in the A10/A13s or older F1C100s SoCs would not support that,
but this code here is purely in the ARMv8/AArch64 code path, so it's
safe to use unconditionally.

Reported-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>