firmware/psci: Pass given partition number through

Pi 5 uses BL31 as its armstub file, so the reset goes via PSCI. Parse
any "reboot" parameter as a partition number to reboot into.
N.B. This code path is only used if reboot mode has been set to warm
or soft.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>