net: stmmac: rk: move speed GRF register offset to private data

net: stmmac: rk: move speed GRF register offset to private data

Move the speed/clocking related GRF register offset into the driver
private data, convert rk_set_reg_speed() to use it and initialise this
member either from the corresponding member in struct rk_gmac_ops, or
the SoC specific initialisation function.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vmqmr-00000007VCV-3Cz8@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>