This adds two new functions, lmb_alloc_addr and
lmb_get_unreserved_size.

lmb_alloc_addr behaves like lmb_alloc, but it tries to allocate a
pre-specified address range. Unlike lmb_reserve, this address range
must be inside one of the memory ranges that has been set up with
lmb_add.

lmb_get_unreserved_size returns the number of bytes that can be
used up to the next reserved region or the end of valid ram. This
can be 0 if the address passed is reserved.

Added test for these new functions.
Reviewed-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NSimon Goldschmidt <simon.k.r.goldschmidt@gmail.com>