riscv: Fix BUILTIN_DTB for sifive and microchip soc

Fix BUILTIN_DTB config which resulted in a dtb that was actually not
built into the Linux image: in the same manner as Canaan soc does,
create an object file from the dtb file that will get linked into the
Linux image.
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/boot/dts/microchip/Makefile

 # SPDX-License-Identifier: GPL-2.0
 dtb-$(CONFIG_SOC_MICROCHIP_POLARFIRE) += microchip-mpfs-icicle-kit.dtb
+obj-$(CONFIG_BUILTIN_DTB) += $(addsuffix .o, $(dtb-y))

arch/riscv/boot/dts/sifive/Makefile

 # SPDX-License-Identifier: GPL-2.0
 dtb-$(CONFIG_SOC_SIFIVE) += hifive-unleashed-a00.dtb \
 			    hifive-unmatched-a00.dtb
+obj-$(CONFIG_BUILTIN_DTB) += $(addsuffix .o, $(dtb-y))