arm: mxs: Fix mkimage invocation

arm: mxs: Fix mkimage invocation

Remove this ad-hoc mkimage invocation in MXS Makefile and replace
it with the standard mkimage rule instead.

This patch fixes recent build issues introduced by the patch
92a655c mkimage: Set up a file size parameter and keep it updated
These build issues could be triggered by building for example the
MX28EVK and the u-boot.sb image:

$ make mx28evk_defconfig
$ make V=1 u-boot.sb
[...]
make -f ./scripts/Makefile.build obj=arch/arm/cpu/arm926ejs/mxs u-boot.sb
  ./tools/mkimage -n arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg -T mxsimage u-boot.sb
./tools/mkimage: Can't open (null): Bad address
arch/arm/cpu/arm926ejs/mxs/Makefile:82: recipe for target 'u-boot.sb' failed
make[1]: *** [u-boot.sb] Error 1
Makefile:989: recipe for target 'u-boot.sb' failed
make: *** [u-boot.sb] Error 2

With this patch:
  ./tools/mkimage -n arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg -T mxsimage -d arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg u-boot.sb

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Juha Lumme <juha.lumme@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>