imx: power-domain: Inline arch-imx8m/power-domain.h

The arch/arm/include/asm/arch-imx8m/power-domain.h is not included
anywhere except in drivers/power/domain/imx8m-power-domain.c, just
inline the content and drop the header. No functional change.

Tested-By: Tim Harvey <tharvey@gateworks.com> #imx8mp-venice-defconfig
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>