platform: starfive: rename "stf,axp15060-regulator" -> "x-powers,axp15060"

OpenSBI uses the device tree compatible string "stf,axp15060-regulator"
for the regulator node. However, the string used by U-Boot (and Linux)
is actually "x-powers,axp15060". As OpenSBI gets the device tree from
U-Boot, this causes the regulator device to be undetected, and OpenSBI
does not use this device to perform board reset/shutdown.

Rename this device tree compatible string to match U-Boot (and Linux).

Signed-off-by: Nam Cao <namcao@linutronix.de>
Acked-by: Minda Chen <minda.chen@starfivetech.com>
Tested-by: Minda Chen <minda.chen@starfivetech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>