Makefile: remove misc include and simple-bin build artifacts on clean

make rock5b-rk3588_defconfig
make
make clean
git status

before
~~~~~~~
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	include/autoconf.mk
	include/autoconf.mk.dep
	include/config.h
	mkimage-in-simple-bin-spi.mkimage-rockchip-tpl
	mkimage-in-simple-bin-spi.mkimage-u-boot-spl
	mkimage-in-simple-bin.mkimage-rockchip-tpl
	mkimage-in-simple-bin.mkimage-u-boot-spl
	simple-bin.fit.fit
	simple-bin.fit.itb

after
~~~~~~~
On branch master
Your branch is ahead of 'origin/master' by 1 commit.
  (use "git push" to publish your local commits)

nothing to commit, working tree clean

Signed-off-by: John Clark <inindev@gmail.com>