mpc85xx: p2020: fix cfi-nor detection

mpc85xx: p2020: fix cfi-nor detection

At this moment p2020rdb has broken images, because NOR memory connected
to eLBC bus isn't detected.

In 642b1e8dbed7 linux tree commit, config dependencies of MTD_PHYSMAP_OF
was changed and now MTD_PHYSMAP is required.

This patch adds MTD_PHYSMAP option to kernel config in p2020 subtarget
and fix booting of p2020rdb.

Fixes: 13b1db795f05 ("mpc85xx: add support for kernel 5.4")
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
(cherry picked from commit 76649fd06d454f1b93f2ee74951ef91225155a8c)