sh: rework memory limits to work with LMB.

This reworks the memory limit handling to tie in through the available
LMB infrastructure. This requires a bit of reordering as we need to have
all of the LMB reservations taken care of prior to establishing the
limits.

While we're at it, the crash kernel reservation semantics are reworked
so that we allocate from the bottom up and reduce the risk of having
to disable the memory limit due to a clash with the crash kernel
reservation.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>