ARM: dts: r9a06g032: describe GMAC2

RZ/N1 SoC includes two MAC named GMACx that are compatible with the
"snps,dwmac" driver. GMAC1 is connected directly to the MII converter
port 1. GMAC2 however can be used as the MAC for the switch CPU
management port or can be muxed to be connected directly to the MII
converter port 2. This commit add description for the GMAC2 which will
be used by the switch description.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>