net: stmmac: rk: use rk_encode_wm16() for RMII clock

net: stmmac: rk: use rk_encode_wm16() for RMII clock

The RMII clock is a single bit, which is set for 100M and clear for
10M. Move this out of struct rk_reg_speed_data (which gets rid of
this structure) into the struct rk_clock_fields as the bitmask for
this bit.

This gets rid of the per-SoC variability in the calls to
rk_set_reg_speed().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vmqnH-00000007VCz-1WmP@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>