i.mx6q: configs: Add fdt_high and initrd_high variables

i.mx6q: configs: Add fdt_high and initrd_high variables

To be able to load the device tree and initrd correctly, set
the fdt_high and initrd_high environment variables.

Using 0xffffffff implies that the device tree and the initrd
are initially copied to working addresses. This will avoid an
additional copy.

Loading the device tree to 0x30000000 and the initrd to 0x3c000000
should work for both boards, the ARM2 and SabreLite.

Example (SabreLite):

fatload mmc 0:2 0x10000000 uImage
fatload mmc 0:2 0x3c000000 uInitrd
fatload mmc 0:2 0x30000000 board.dtb
bootm 0x10000000 0x3c000000 0x30000000

Note: This requires that the kernel has CONFIG_HIGHMEM enabled.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
CC: Jason Liu <jason.hui@linaro.org>
CC: Stefano Babic <sbabic@denx.de>
Acked-by: Jason Liu <jason.hui@linaro.org>