mtd: ofpart: Check availability of reg property instead of name property

The previous implementation breaks the dts binding "mtd-physmap.txt". This
implementation fixes the issue by checking the availability of the reg
property instead of the name property.

Cc: stable@kernel.org
Signed-off-by: Benjamin Krill <ben@codiert.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>