ARM: dts: BCM5301X: Use corretc pinctrl compatible for 4709x

BCM47094 version of pinmux uses different compatible and supports MDIO
pinmux pins. Hence, use the correct compatible string and defines the
MDIO pins group.

Signed-off-by: Vivek Unune <npcomplete13@gmail.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>