test: lmb: Rework lib_test_lmb_max_regions test to scale

First, this test depends on CONFIG_LMB_USE_MAX_REGIONS, so add that as a
test before building. Second, instead of using a hard-coded value of 8,
which is the default of CONFIG_LMB_USE_MAX_REGIONS previously, use that
directly and update the comments. The only trick here is that one part
of the test itself also was written with the value of 8 itself in mind.
Rework the size of the lmb region we allocate to scale with the value of
CONFIG_LMB_USE_MAX_REGIONS.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>