[POWERPC] Update MTD OF documentation

This updates the Documentation/powerpc part of the MTD OF
implementation with the new field probe-type. Its support has already
been implemented in MTD part (drivers/mtd/maps/physmap_of.c).

Signed-off-by: Vitaly Wool <vwool@ru.mvista.com>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>