riscv: Allow setting custom mvendorid, marchid, and mimpid

We add command-line parameter to set custom mvendorid, marchid, and
mimpid so that users can show fake CPU type to Guest/VM which does
not match underlying Host CPU.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Link: https://lore.kernel.org/r/20230712163501.1769737-4-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>