mb862xxfb: use CONFIG_OF instead of CONFIG_PPC_OF

With this change, the driver builds fine on Microblaze, which helps
allyesconfig compile tests.

I did not test sparc, but the change should have the same effect there.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Anatolij Gustschin <agust@denx.de>
Tested-by: Anatolij Gustschin <agust@denx.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>