ATSTK1002: Use SDRAM + 4MB as default load address

ATSTK1002: Use SDRAM + 4MB as default load address

Many people run into problems when they compile a big kernel and load
the uImage at the default SDRAM + 2MB address as the kernel will
overwrite the uImage as it is being unpacked. Increase the default
load address so that we can load a 4MB kernel image without any
problems.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>