cache: Check dcache availability before calling cache functions

cache: Check dcache availability before calling cache functions

When the data cache (dcache) is disabled, calling related
status functions can lead to compilation errors due to
undefined references.

Adding a !CONFIG_IS_ENABLED(SYS_DCACHE_OFF) check before
invoking dcache_status() (used in common/memsize.c:get_ram_size())
and mmu_status() (from arch/arm/include/asm/io.h).

Without this check, builds with dcache disabled will fail to compile.

Signed-off-by: Boon Khai Ng <boon.khai.ng@altera.com>
Reviewed-by: Tom Rini <trini@konsulko.com>