uboot-imx

spl: fix Makefile for NOR, XIP and YMODEM

During the the conversion to $(SPL_TPL_), the SPL_ fragment was
left over for the NOR, XIP and YMODEM boot methods in SPL, making
these unselectable.

This commit fixes this by dropping the spurious 'SPL_' fragment
from each line.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reported-by: Bo Shen <voice.shen@gmail.com>
Fixes: f94e643 (spl: consistently use $(SPL_TPL_) to select features for SPL and TPL builds)