mb862xxfb: restrict compliation of platform driver to PPC

The OpenFirmware part of this driver is uncompilable on SPARC due to it's
dependance on several PPC specific functions.

Restricting this to PPC to prevent these build errors:
  CC      drivers/video/mb862xx/mb862xxfb.o
drivers/video/mb862xx/mb862xxfb.c: In function 'of_platform_mb862xx_probe':
drivers/video/mb862xx/mb862xxfb.c:559: error: implicit declaration of function 'of_address_to_resource'
drivers/video/mb862xx/mb862xxfb.c:575: error: 'NO_IRQ' undeclared (first use in this function)
drivers/video/mb862xx/mb862xxfb.c:575: error: (Each undeclared identifier is reported only once
drivers/video/mb862xx/mb862xxfb.c:575: error: for each function it appears in.)

This was found using randconfig builds.

Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>