ARM: AM33xx: PRM: Implement REBOOT_COLD

Historically AM33xx performed warm software reset even though requested
(and default) was REBOOT_COLD. Reflect the de-facto default mode in
/sys/kernel/reboot/mode correctly and implement the real REBOOT_COLD
(if configured explicitly).

Tested-by: Matthias Michel <matthias.michel@siemens.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Message-ID: <20240221154614.3549951-2-alexander.sverdlin@siemens.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>