drm/simpledrm: Add support for multiple "power-domains"

drm/simpledrm: Add support for multiple "power-domains"

Multiple power domains need to be handled explicitly in each driver. The
driver core can not handle it automatically since it is not aware of
power sequencing requirements the hardware might have. This is not a
problem for simpledrm since everything is expected to be powered on by
the bootloader. simpledrm has just ensure it remains powered on during
its lifetime.
This is required on Apple silicon M2 and M2 Pro/Max/Ultra desktop
systems. The HDMI output initialized by the bootloader requires keeping
the display controller and a DP phy power domain on.

Signed-off-by: Janne Grunau <j@jannau.net>
Reviewed-by: Eric Curtin <ecurtin@redhat.com>
Reviewed-by: Neal Gompa <neal@gompa.dev>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230912-simpledrm-multiple-power-domains-v2-1-01b66bfb1980@jannau.net