board: bcm96846: Enable NAND options

board: bcm96846: Enable NAND options

This adds reasonable NAND options to the BCM96846 reference design:

- CMD_NAND, MTD_RAW_NAND
- Disable SYS_NAND_ONFI_DETECTION as this just give error messages
- MTD, MTDPARTS with DM and related config options
- CMD_UBI and CMD_UBIFS as this is likely used with ubi/ubifs

What I didn't add was something like the following:

CONFIG_MTDPARTS_DEFAULT="nand0:256k(cfi),257024k(image)"

Because I don't actually have a BCM96846 reference design. These
are only available to Broadcom and their customers  I think,
but perhaps the people at Broadcom can provide the detail of
the flash layout for BCM96846 so we can add this too so the
bcm96846_config is usable out of the box.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>