net: stmmac: rk: add GMAC_CLK_xx constants, simplify RGMII definitions

net: stmmac: rk: add GMAC_CLK_xx constants, simplify RGMII definitions

All the definitions of the RGMII related xxx_GMAC_CLK_xxx definitions
use the same field values to select the clock rate. Provide common
definitions for these field values, passing them in to a single macro
for each variant that generates the appropriate values for the speed
register.

No change to produced code on aarch64.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/E1vl3G0-00000006v3R-2ZBc@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>