pmdomain: bcm: bcm2835-power: Prepare to support BCM2712

pmdomain: bcm: bcm2835-power: Prepare to support BCM2712

BCM2712 has a PM block but lacks asb and rpivid_asb register
spaces. To avoid unwanted results add a check for asb existence
during probe and also add a new register offset for bcm2712 to
control grafx_v3d power domain. The decision to use the new
register is implicit - if asb register base is null then the
driver is probed for bcm2712 (the other supported SoCs have
asb register space).

Signed-off-by: Stanimir Varbanov <svarbanov@suse.de>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>