AM335X: Set fdt_high for AM335X devices to enable booting with Device Tree

AM335X: Set fdt_high for AM335X devices to enable booting with Device Tree

For AM335X boards, such as the EVM and Bone 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 0xffffffff (to prevent the copy)
the kernel is able to locate the DT blob and boot.

This patch is tested on BeagleBone platform.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Cc: Tom Rini <trini@ti.com>