sh: Do not try merging two 128MB PMB mappings

There is a logic error in pmb_merge() that means we will incorrectly try
to merge two 128MB PMB mappings into one mapping. However, 256MB isn't a
valid PMB map size and pmb_merge() will actually drop the second 128MB
mapping.

This patch allows my SDK7786 board to boot when configured with
CONFIG_MEMORY_SIZE=0x10000000.

Signed-off-by: Matt Fleming <matt@console-pimps.org>