powerpc/config: Renable MTD_PHYSMAP_OF

powerpc/config: Renable MTD_PHYSMAP_OF

[ Upstream commit d0e28a6145c3455b69991245e7f6147eb914b34a ]

CONFIG_MTD_PHYSMAP_OF is not longer enabled as it depends on
MTD_PHYSMAP which is not enabled.

This is a regression from commit a229262e82b6 ("mtd: maps: Merge
physmap_of.c into physmap-core.c"), which added the extra dependency.
Add CONFIG_MTD_PHYSMAP=y so this stays in the config, as Christophe said
it is useful for build coverage.

Fixes: a229262e82b6 ("mtd: maps: Merge physmap_of.c into physmap-core.c")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210817045407.2445664-3-joel@jms.id.au
Signed-off-by: Sasha Levin <sashal@kernel.org>