ath79: gmac: ar934x: Add parser for mii-gmac0-slave

ath79: gmac: ar934x: Add parser for mii-gmac0-slave

While converting Nanostation M XW from current ar71xx code to ath79 I've
hit one issue, where the ethernet networking wasn't working, so I was
checking every bit in the networking setup path between ar71xx and
ath79.

I've came to the following code in ar71xx/mach-ubnt-xm.c:

 static void __init ubnt_xw_init(void) {
 	...
 	ath79_setup_ar934x_eth_cfg(AR934X_ETH_CFG_MII_GMAC0 |
                                   AR934X_ETH_CFG_MII_GMAC0_SLAVE);
 	...
 }

Where this code is setting AR934X_ETH_CFG_MII_GMAC0_SLAVE bit in
AR934X_GMAC_REG_ETH_CFG register, but I couldn't find a way of setting
this bit from DTS, so this patch adds `mii-gmac0-slave` DTS property
which allows setting of this bit in `gmac-config`, which is then used in
Nanostation M XW DTS.

Tested-by: Joe Ayers <ae6xe@arrl.net>
Signed-off-by: Petr Štetiar <ynezz@true.cz>