board/qemu/riscv64-virt: update qemu cmdline for Qemu >= 5.1

board/qemu/riscv64-virt: update qemu cmdline for Qemu >= 5.1

Since 52f188140cd28e90103edf67db6c2cabb979f5d6 (qemu version bump to 5.1),
the image generated by qemu_riscv64_virt_defconfig doesn't boot anyore with
the following error:

rom: requested regions overlap (rom phdr #0: [...]/images//fw_jump.elf. free=0x000000008000e240, addr=0x0000000080000000)
qemu-system-riscv64: rom check and register reset failed

Update the qemu command line as described in the Qemu wiki for riscv64 [1]

Fixes:
https://gitlab.com/jugurthaB/buildroot/-/jobs/686104707

[1] https://wiki.qemu.org/Documentation/Platforms/RISCV#Booting_64-bit_OpenEmbedded_Images

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Jugurtha BELKALEM <jugurtha.belkalem@smile.fr>
Cc: Alistair Francis <alistair.francis@wdc.com>
Cc: Mark Corbin <mark@dibsco.co.uk>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>