ARM: OMAP2+: Remove legacy booting support for n8x0

Now we can boot n8x with the appended device tree with:

$ ARCH=arm CROSS_COMPILE=/usr/bin/arm-linux-gnueabi- make omap2420-n800.dtb
$ cat arch/arm/boot/zImage arch/arm/boot/dts/omap2420-n800.dtb > /tmp/zImage

Note that you need at least the following enabled:

CONFIG_ARM_APPENDED_DTB=y
CONFIG_ARM_ATAG_DTB_COMPAT=y
CONFIG_ARM_ATAG_DTB_COMPAT_CMDLINE_FROM_BOOTLOADER=y
CONFIG_PINCTRL=y
CONFIG_PINCTRL_SINGLE=y

Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>