bcm2835_pinctrl: Probe pre-reloc

The serial drivers now depend on the pinctrl driver to determine whether
they are enabled. That means if a serial device wants to be used pre-reloc,
we also need the pinctrl device pre-reloc.

Adapt the pinctrl driver as well as dts overlay accordingly.

Signed-off-by: Alexander Graf <agraf@suse.de>