net: stmmac: rk: use rk_encode_wm16() for RGMII clocks

net: stmmac: rk: use rk_encode_wm16() for RGMII clocks

As all of the RGMII clock selection bitfields (gmii_clk_sel) use the
same encoding, parameterise this by providing the bitfield mask in
the BSP private data.

This is the last user of GRF_FIELD_CONST(), so remove that definition
as well.

One additional change is for RK3328 - as only gmac2io supports RGMII,
only initialise the mask for this instance.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vmqn7-00000007VCn-0OZA@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>