lmb: Always compile arch_lmb_reserve() into U-Boot on arc

lmb: Always compile arch_lmb_reserve() into U-Boot on arc

The arch_lmb_reserve() is called by lib/lmb.c lmb_reserve_common() even
if CMD_BOOTM is not enabled. However, the arc variant of arch_lmb_reserve()
is only compiled in if CMD_BOOTM is enabled.

This currently does not trigger build error, because there is an empty
weak implementation of arch_lmb_reserve(), however that is not the
function that should be used on arc.

Fix this by moving the arch_lmb_reserve() implementation into common
code and always compile it in.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Tom Rini <trini@konsulko.com>