ARC: [plat-arcfpga] Add defconfig without initramfs location

This is useful if you want to build a kernel without a ramfs, or
if you want to test the kernel build without having an initramfs
available at the hardcoded location.

Cc: Vineet Gupta <vgupta@synopsys.com>
Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>