powerpc/config: Renable MTD_PHYSMAP_OF

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

This is a regression from commit 642b1e8dbe ("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: 642b1e8dbe ("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

arch/powerpc/configs/mpc885_ads_defconfig

@@ -33,6 +33,7 @@ CONFIG_MTD_CFI_GEOMETRY=y
 # CONFIG_MTD_CFI_I2 is not set
 CONFIG_MTD_CFI_I4=y
 CONFIG_MTD_CFI_AMDSTD=y
+CONFIG_MTD_PHYSMAP=y
 CONFIG_MTD_PHYSMAP_OF=y
 # CONFIG_BLK_DEV is not set
 CONFIG_NETDEVICES=y