arm64: kernel: use ordinary return/argument register for el2_setup()

arm64: kernel: use ordinary return/argument register for el2_setup()

The function el2_setup() passes its return value in register w20, and
in the two cases where the caller actually cares about this return value,
it is passed into set_cpu_boot_mode_flag() [almost] directly, which
expects its input in w20 as well.

So there is no reason to use a 'special' callee saved register here, but
we can simply follow the PCS for return value and first argument,
respectively.

Reviewed-by: Mark Rutland <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Signed-off-by: Will Deacon <[email protected]>