MIPS: AR7: Fix build warning on memory.c

This patch fixes the following build warning:
arch/mips/ar7/memory.c: In function 'memsize':
arch/mips/ar7/memory.c:55: warning: passing argument 1 of 'writel' makes integer from pointer without a cast

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>