lmb: add a max parameter in the struct lmb_region

Add a max parameter in lmb_region struct to handle test
in lmb_add_region without using the MAX_LMB_REGIONS
define.

This patch allows to modify these size independently for
memory of reserved regions in the next patches.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>