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>

configs/bcm96846_defconfig

@@ -18,5 +18,17 @@ CONFIG_OF_STDOUT_VIA_ALIAS=y
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_CACHE=y
+CONFIG_CMD_NAND=y
+CONFIG_CMD_UBI=y
+CONFIG_CMD_UBIFS=y
+CONFIG_CMD_BOOTZ=y
+CONFIG_CMD_MTDPARTS=y
 CONFIG_OF_EMBED=y
 CONFIG_CLK=y
+CONFIG_MTD=y
+CONFIG_MTDIDS_DEFAULT="nand0=nand0"
+CONFIG_DM_MTD=y
+CONFIG_MTD_RAW_NAND=y
+CONFIG_MTD_UBI_FASTMAP=y
+CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT=1
+CONFIG_SYS_NAND_ONFI_DETECTION=n