realtek: rtl931x: Fix unsafe MAC_L2_GLOBAL_CTRL2 access

realtek: rtl931x: Fix unsafe MAC_L2_GLOBAL_CTRL2 access

Registers must not be accessed in parallel by multiple drivers.
Read-modify-write operations are not atomic, and the result of parallel
access is undefined.

The MAC_L2_GLOBAL_CTRL2 register is essentially a pin configuration
register and is represented by a pinmux node in the devicetree.  Operations
on this register by the realtek,rtl838x-eth driver must therefore also be
reflected in the devicetree.

Since the MDIO sets used are board-specific, the pins must be enabled in
the board’s devicetree.  This can be achieved using the pinctrl properties
for the realtek,rtl83xx-switch.

    &switch0 {
    	pinctrl-names = "default";
    	pinctrl-0 = <&pinmux_enable_mdc_mdio_0>,
    		    <&pinmux_enable_mdc_mdio_1>;
    	....
    };

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Link: https://github.com/openwrt/openwrt/pull/19815
Signed-off-by: Robert Marko <robimarko@gmail.com>