phy: allwinner: phy-sun6i-mipi-dphy: Add a variant power-on hook

phy: allwinner: phy-sun6i-mipi-dphy: Add a variant power-on hook

The A100 variant uses the same values for the timing registers, and it
uses the same final power-on sequence, but it needs a different analog
register configuration in the middle. Support this by moving the
variant-specific parts to a hook provided by the variant.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20221114022113.31694-8-samuel@sholland.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>