MIPS: ath79: replace ath724x to ar724x

Replace the 'ath724x' to 'ar724x' in function, variable and
structure names to reflect the name of the real SoC.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: René Bolldorf <xsecute@googlemail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3490/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>