arch/sh: add sh7786_mm_sel() function

The SH7786 has different physical memory layout configurations,
configurable through the MMSELR register. The configuration is
typically defined by the bootloader, so Linux generally doesn't care.

Except that depending on the configuration, some PCI MEM areas may or
may not be available. This commit adds a helper function that allows
to retrieve the current physical memory layout configuration. It will
be used in a following patch to exclude unusable PCI MEM areas during
the PCI initialization.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Rich Felker <dalias@libc.org>