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 <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Simon Goldschmidt <[email protected]>
Cc: Tom Rini <[email protected]>