powerpc/ps3: ps3_defconfig updates

Update ps3_defconfig.

Sets these options:

  CONFIG_PS3_VRAM=m
  CONFIG_BLK_DEV_DM=m
  CONFIG_USB_HIDDEV=y
  CONFIG_EXT4_FS=y

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>