drm/bridge: sii902x: Enable I/O and core VCC supplies if present

On the SII9022, the IOVCC and CVCC12 supplies must reach the correct
voltage before the reset sequence is initiated. On most boards, this
assumption is true at boot-up, so initialization succeeds.

However, when we try to initialize the chip with incorrect supply
voltages, it will not respond to I2C requests. sii902x_probe() fails
with -ENXIO.

To resolve this, look for the "iovcc" and "cvcc12" regulators, and
make sure they are enabled before starting the reset sequence. If
these supplies are not available in devicetree, then they will default
to dummy-regulator. In that case everything will work like before.

This was observed on a STM32MP157C-DK2 booting in u-boot falcon mode.
On this board, the supplies would be set by the second stage
bootloader, which does not run in falcon mode.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
[Fix checkpatch warnings]
Link: https://patchwork.freedesktop.org/patch/msgid/20201020221501.260025-2-mr.nuke.me@gmail.com