ls-ddr-phy: select by default for layerscape/armv8_64b

Currently, ls-ddr-phy package is being included in the individual
profile DEVICE_PACKAGES but using the feature that allows skipping their
inclusion in the end image package list if prefixed with a tilde(~) which
was added in:
377b66990b97 ("build: introduce support to declare skip package")

But it not added to Image Builder so currently trying to build layerscape
device images in Image Builder will fail with:
ERROR: '~ls-ddr-phy' is not a valid world dependency, format is name(@tag)([<>~=]version)

So, instead of having to rely on support for skipping package installation
and declaring the ls-ddr-phy package in DEVICE_PACKAGES lets select it when
layerscape/armv8_64b target is selected.

Fixes: #18412
Link: https://github.com/openwrt/openwrt/pull/18462
Signed-off-by: Robert Marko <robimarko@gmail.com>

package/firmware/layerscape/ls-ddr-phy/Makefile

@@ -27,6 +27,8 @@ define Package/layerscape-ddr-phy
   CATEGORY:=Firmware
   TITLE:=NXP Layerscape DDR PHY firmware
   DEPENDS:=@TARGET_layerscape
+  DEFAULT:=y if TARGET_layerscape_armv8_64b
+  HIDDEN:=1
 endef
 
 define Build/Compile