MIPS: ath79: make use of the new memory detection code

There is now a generic function for detecting memory size. Use this instead of
the one found in the ath79 support.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5149/