[U-Boot] spl: fix Makefile for NOR, XIP and YMODEM

[Philipp Tomsich]

> 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 at theobroma-systems.com>
> Reported-by: Bo Shen <voice.shen at gmail.com>
> Fixes: f94e643 (spl: consistently use $(SPL_TPL_) to select features for SPL and TPL builds)
> ---
> 
>  common/spl/Makefile | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 

Applied to u-boot-rockchip, thanks!