of: separate out "generic" memory bank adding ...

This patch separates out the "generic" memory
segment registration function (of_add_memory_bank())
from of_add_memory().
The MIPS architecture has different view on memory
resources than the ARM and PPC architectures
so the "generic" of_add_memory_bank() is
unusable for the MIPS architecture.
We can add MIPS-specific of_add_memory_bank()
into arch/mips code.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>