OMAP4: Set fdt_high for OMAP4 devices to enable booting with Device Tree

OMAP4: Set fdt_high for OMAP4 devices to enable booting with Device Tree

For OMAP4 boards, such as the panda-es, that have 1GB of memory the linux
kernel fails to locate the device tree blob on boot. The reason being is that
u-boot is copying the DT blob to the upper part of RAM when booting the kernel
and the kernel is unable to access the blob. By setting the fdt_high variable
to either 0xffffffff (to prevent the copy) or 0xac000000 (704MB boundary
of memory for OMAP4) the kernel is able to locate the DT blob and boot.

Based upon following patch by Dirk Behme set the fdt_high variable to allow
booting with device tree on OMAP4 boards.

"7e9603e i.mx6q: configs: Add fdt_high and initrd_high variables"

Cc: Sricharan R <r.sricharan@ti.com>
Cc: Sandeep Paulraj <s-paulraj@ti.com>
Cc: Tom Rini <trini@ti.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>