xtensa: don't make bootmem bitmap larger than required

If min_low_pfn is non-zero, the bitmap reserved for bootmem is bigger
than needed.  The number of pages bootmem has to maintain is the range
from min_low_pfn to max_low_pfn.

For now it has only been a theoretical mistake, min_low_pfn was always
zero.

Signed-off-by: Johannes Weiner <jw@emlix.com>
Signed-off-by: Chris Zankel <chris@zankel.net>