xtensa: nommu: reserve memory below PLATFORM_DEFAULT_MEM_START

Memory accounting code can't handle pages below
PLATFORM_DEFAULT_MEM_START. Reserve those pages if they exist.
When PLATFORM_DEFAULT_MEM_START is zero reserve one page at address 0 to
make sure that successfull memory allocations don't return NULL.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>