of: make unittest select OF_EARLY_FLATTREE instead of depend on it

The DT unittest currently requires an arch (typically) to select
OF_EARLY_FLATTREE. Remove this dependency by selecting it directly so that
the unittest can be enabled easily on any architecture. With this and the
prior commit, we can easily enable and run unittests starting with x86
defconfig rather than hunting for the combination of config options to
enable OF on x86.
Signed-off-by: NRob Herring <robh@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: NGrant Likely <grant.likely@lianro.org>

drivers/of/Kconfig

@@ -13,7 +13,8 @@ if OF
 
 config OF_UNITTEST
 	bool "Device Tree runtime unit tests"
-	depends on OF_IRQ && OF_EARLY_FLATTREE
+	depends on OF_IRQ
+	select OF_EARLY_FLATTREE
 	select OF_RESOLVE
 	help
 	  This option builds in test cases for the device tree infrastructure