ARM: fix arch/arm/dts/Makefile

Since commit 27cb7300 ("Ensure device tree DTS is compiled"),
build succeeds irrespective of the correctness of Makefile.

In fact, you can compile any defconfig without adding any entry in
arch/*/dts/Makefile.

As a result, a lot of wrong code have been merged unnoticed.

I am going to revert that commit, and lots of hidden issues have
come to light:

[1] Typos

  armada-3720-uDPU.dts, sun8i-a83t-tbs-a711.dts

  use the extension ".dts" instead of ".dtb"

[2] DTB is associated to undefined CONFIG option

  For example, mx6sllevk_defconfig defines CONFIG_MX6SLL, but
  associates its device tree to CONFIG_MX6SL, which is undefined.

[3] Lots of entries are missing

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
[trini: add imx6ul pico dtbs]
Signed-off-by: Tom Rini <trini@konsulko.com>