phy: qcom-qmp-pcie: add register init helper

Generalise the serdes initialisation helper so that it can be used to
initialise all the PHY registers (e.g. serdes, tx, rx, pcs).

Note that this defers the ungating of the PIPE clock somewhat, which is
fine as it isn't needed until starting the PHY.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>