phy: add support for AM654x SERDES

phy: add support for AM654x SERDES

Add a new SERDES driver for TI's AM654x SoC which configures
the SERDES only for PCIe. Support fo USB3 can be added later.

SERDES in am654x has three input clocks (left input, external
reference clock and right input) and two output clocks (left
output and right output) in addition to a PLL mux clock which
the SERDES uses for Clock Multiplier Unit (CMU refclock).

The PLL mux clock can select from one of the three input
clocks. The right output can select between left input and
external reference clock while the left output can select
between the right input and external reference clock.

The driver has support to select PLL mux and left/right output
mux as specified in device tree.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>