mtd-physmap: add support users can assign the probe type in board files

There are three reasons to add this support:
1. users probably know the interface type of their flashs, then probe
can be faster if they give the right type in platform data since wrong
types will not be detected.
2. sometimes, detecting can cause destory to system. For example, for
kernel XIP, detecting can cause NOR enter a mode instructions can not
be fetched right, which will make kernel crash.
3. For a new probe which is not listed in the rom_probe_types, if users
assign it in board files, physmap can still probe it.

Signed-off-by: Barry Song <21cnbao@gmail.com>
Signed-off-by: Mike Frysinger <vapier.adi@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>