ARM: dts: sama7g5ek: fix TXC pin configuration

TXC line is directly connected from the SoC to the KSZ9131 PHY. There
is a transient state on this signal, before configuring it to RGMII,
which leads to packet transmit being blocked.
Keeping a pull-up when muxing this pin as function A (G0_TXCK) fixes
the issue.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>