riscv: qemu: Remove fdt_high default

riscv: qemu: Remove fdt_high default

Setting fdt_high to all ones is discouraged and does not appear to be
useful for RISC-V QEMU. Moreover, it causes a boot failure when the FDT
generated internally by QEMU is used while booting. Remove it to allow
U-Boot to pick a suitable address and relocate the FDT.

Closes: https://lore.kernel.org/u-boot/8397369a-9b0b-4798-9c30-3a81165657d6@iscas.ac.cn
Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>