LMB: Fix undefined lmb_reserve() on non-lmb platforms

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Stephen Warren <swarren@nvidia.com>
Cc: Tom Warren <TWarren@nvidia.com>
Cc: Graeme Russ <graeme.russ@gmail.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>