dts: add property removal option CONFIG_OF_REMOVE_PROPS

This can be used for device tree size reduction similar as
CONFIG_OF_SPL_REMOVE_PROPS option. Some boards must pass the
built-in DTB unchanged to the kernel, thus we may not cut it
down unconditionally. Therefore enable the property removal
list option only if CONFIG_OF_DTB_PROPS_REMOVE is selected.

Signed-off-by: Anatolij Gustschin <agust@denx.de>