MIPS: Add dtbs_install target

Add the dtbs_install Makefile target to install the dtb files into
$INSTALL_DTBS_PATH.

Signed-off-by: Andrew Bresticker <[e-mail]>
Tested-by: Kevin Cernekee <[e-mail]>
Cc: [e-mail]
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Patchwork: https://patchwork.linux-mips.org/patch/8836/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/Makefile

···
 dtbs: scripts
 	$(Q)$(MAKE) $(build)=arch/mips/boot/dts
 
+PHONY += dtbs_install
+dtbs_install:
+	$(Q)$(MAKE) $(dtbinst)=arch/mips/boot/dts
+
 archprepare:
 ifdef CONFIG_MIPS32_N32
 	@echo '  Checking missing-syscalls for N32'
···
 	echo '  uImage.lzma          - U-Boot image (lzma)'
 	echo '  uImage.lzo           - U-Boot image (lzo)'
 	echo '  dtbs                 - Device-tree blobs for enabled boards'
+	echo '  dtbs_install         - Install dtbs to $(INSTALL_DTBS_PATH)'
 	echo
 	echo '  These will be default as appropriate for a configured platform.'
 endef