MIPS: ath79: Rename ATH79_DEV_AR913X_WMAC option to ATH79_DEV_WMAC

The ATH79_DEV_AR913X_WMAC option was used to select the AR913x specific
wireless MAC registration code.  The registration code now supports the
AR933X SoCs as well. Rename the option to reflect the changes.

Also make the new option depends on SOC_AR933X.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3031/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>