MFC r327222:

Add a new ARM kernel option, LOCORE_MAP_MB, to control the size of the
kernel VA mapping in the temporary page tables set up by locore-v6.S.

The number used to be hard-coded to 64MB, which is still the default if
the kernel option is not specified. However, 64MB is insufficient for
using a large mdroot filesystem. The hard-coded number can't be safely
increased because too large a number may run into memory-mapped IO space
on some SoCs that must not be mapped as ordinary memory.