hw/loongarch: Add fdt support

Add LoongArch flatted device tree, adding cpu device node, firmware cfg node,
pcie node into it, and create fdt rom memory region. Now fdt info is not
full since only uefi bios uses fdt, linux kernel does not use fdt.
Loongarch Linux kernel uses acpi table which is full in qemu virt
machine.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Xiaojuan Yang <yangxiaojuan@loongson.cn>
Message-Id: <20220712083206.4187715-7-yangxiaojuan@loongson.cn>
[rth: Set TARGET_NEED_FDT, add fdt to meson.build]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>