openrisc: mm/init.c: remove unused memblock_region variable in map_ram()

Kernel test robot reports:

cppcheck possible warnings: (new ones prefixed by >>, may not real problems)

>> arch/openrisc/mm/init.c:125:10: warning: Uninitialized variable: region [uninitvar]
            region->base, region->base + region->size);
            ^

Replace usage of memblock_region fields with 'start' and 'end' variables
that are initialized in for_each_mem_range() and remove the declaration of
region.

Fixes: b10d6bca ("arch, drivers: replace for_each_membock() with for_each_mem_range()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>