ARM: dts: am43xx: add support for parallel NAND flash

This patch:
 - enables GPMC h/w and ELM h/w engine for AM43xx devices (am4372.dtsi)
 - adds pinmux and DT node for Micron 4K-paged x8 NAND device (MT29F4G08AB)
   present on following boards:

   am43x-epos-evm:
	On this board, NAND Flash control lines are muxed with QSPI, Thus only
        one of the two can be used at a time. Selection is controlled by:
        (a) dynamically driving following GPIO pin from software
            GPMC_A0(GPIO) == 0 NAND is selected (default)
            GPMC_A0(GPIO) == 1 eMMC is selected

Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>