lmb: add a flag to allow suppressing memory map change notification

Add a flag LMB_NONOTIFY that can be passed to the LMB API's for
reserving memory. This will then result in no notification being sent
from the LMB module for the changes to the LMB's memory map.

While here, also add a description of the memory attributes that the
flags signify.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>