lmb: Fix LMB_MEMORY_REGIONS flag usage

lmb: Fix LMB_MEMORY_REGIONS flag usage

Remove test on CONFIG_LMB_MEMORY_REGIONS introduced by commit
7c1860fce4e3 ("lmb: Fix lmb property's defination under struct lmb").

This code in lmb_init() is strange, because if CONFIG_LMB_USE_MAX_REGIONS
and CONFIG_LMB_MEMORY_REGIONS are not defined, the implicit #else is empty
and the required initialization is not done:
lmb->memory.max = ?
lmb->reserved.max = ?

But this setting is not possible:
- CONFIG_LMB_USE_MAX_REGIONS not defined
- CONFIG_LMB_MEMORY_REGIONS not defined
because CONFIG_LMB_MEMORY_REGIONS and CONFIG_LMB_RESERVED_REGIONS are
defined as soon as the CONFIG_LMB_USE_MAX_REGIONS is not defined.

This patch removes this impossible case #elif and I add some
explanation in lmb.h to explain why in the struct lmb {} the lmb
property is defined if CONFIG_LMB_MEMORY_REGIONS is NOT defined.

This patch also removes CONFIG_LMB_XXX dependency on CONFIG_LMB as these
defines are used in API file lmb.h and not only in library file.

Fixes: 5e2548c1d6e03 ("lmb: Fix LMB_MEMORY_REGIONS flag usage")
Reported-by: Mark Millard <marklmi@yahoo.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Michal Simek <michal.simek@amd.com>