board: ge: bx50v3: cleanup phy config

The current PHY rework does the following things:

1. Configure 125MHz clock
2. Setup the TX clock delay (RX is enabled by default),
3. Setup reserved bits to avoid voltage peak

The clock delays are nowadays already configured by the
PHY driver (in ar803x_delay_config). The code for that
can simply be dropped. The clock speed can also be
configured by the PHY driver by adding the device tree
property "qca,clk-out-frequency".

What is left is setting up the undocumented reserved bits
to avoid the voltage peak problem. I slightly improved its
documentation while updating the board's PHY rework code.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>