ARM: ixp4xx_defconfig: add MTD_PHYSMAP

Without MTD_PHYSMAP/MTD_PHYSMAP_OF the flash partitions of my welltech,epbx100
are not detected.

Link: https://lore.kernel.org/r/20210504193457.4008384-3-clabbe@baylibre.com


Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Olof Johansson <olof@lixom.net>