cmd: cache: Remove weak functions

It should be up to an architecture to decide how to implement cache
functions, and if they need to use weak functions or not. Allowing the
cache command to be built without cache functionality implemented is
unhelpful. Further, guard the call to noncached_set_region with
CONFIG_SYS_NONCACHED_MEMORY as that's when it's implemented and again is
an architecture specific detail.

Signed-off-by: Tom Rini <trini@konsulko.com>