ARM: dts: da850-evm: Fix MDIO pinmux

In attempts to speed up SPL and reduce size, the MDIO pin muxing
was inadvertently affected.  Since the ethernet driver will setup
the pin muxing when ethernet is loaded, this patch will also
pinmux the MDIO pins at the same time.  Once an DM compatible
MDIO driver is available, this  can be removed.

Fixes: 877ab242 ("ARM: davinci: da850: Manual pinmux only
when PINCTRL not available")

Signed-off-by: Adam Ford <aford173@gmail.com>