kbuild: Simplify builds with CONFIG_OF_ALL_DTBS

We update 'always-y' based on CONFIG_OF_ALL_DTBS three times. It would be
far more straight forward if we rather update dtb-y to include all .dtb
files if CONFIG_OF_ALL_DTBS is enabled.

Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Frank Rowand <frank.rowand@sony.com>
Tested-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/7fe7e5ef6ed75450ddf6c224b8adb53059e504e2.1615354376.git.viresh.kumar@linaro.org