Makefile: Tidy up SPL dtb production

Use the SPL_NAME variable to simplify the rules. Drop the SPL targets
clean-files since the SPL and TPL dts/ directories are removed by
existing rules. Move the SPL rules into a new spl_dtbs to avoid the
complicated $(if) construct.

Also drop unused pieces from the 'targets' variable.

With this, SPL and TPL have separate dtbs which respect the various
u-boot,dm-spl / u-boot,dm-tpl tags.

Signed-off-by: Simon Glass <sjg@chromium.org>