MIPS: ath79: Increase max memory limit to 256MByte

At least QCA955x can handle up to 256MBytes.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Cc: linux-mips@linux-mips.org
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: Helmut Schaa <helmut.schaa@googlemail.com>
Patchwork: https://patchwork.linux-mips.org/patch/8738/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>