rockchip: clk: rk3399: implement set_parent() operation

This implements the (newly added) set_parent() operation for the
RK3399 with a focus on allowing the RGMII clock parent to be
configured via the assigned-clock-parents property of the GMAC node.

This implementation supports only the GMAC (in fact only the RGMII
clock parent) and allows to set this clock's parent either to the
internal SCLK_GMAC or to an external clock input (identifiable by it
providing a 'clock-output-name' of "gmac_clkin").

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: David Wu <david.wu@rock-chips.com>

Series-changes: 2
- Fixed David's email address.