ARM: dts: am437x-gp-evm: add support for parallel NAND flash

Adds pinmux and DT node for Micron (MT29F4G08AB) x8 NAND device present on
am437x-gp-evm board.
(1) As NAND Flash data lines are muxed with eMMC, Thus at a given time either
    eMMC or NAND can be enabled. Selection between eMMC and NAND is controlled:
    (a) By dynamically driving following GPIO pin from software
        SPI2_CS0(GPIO) == 0 NAND is selected (default)
        SPI2_CS0(GPIO) == 1 eMMC is selected
    (b) By statically using Jumper (J89) on the board

(2) As NAND device connnected to this board has page-size=4K and oob-size=224,
    So ROM code expects boot-loaders to be flashed in BCH16 ECC scheme for
    NAND boot.

Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>