uboot-imx

Remove nios-32 arch remnants

nios-32 arch was removed back in 2010 (1117cbf). Code depending on
its headers (nios.h, nios-io.h) can't possibly compile since then.
As it wasn't fixed till now it is safe to remove.

Signed-off-by: Vasili Galka <vvv444@gmail.com>