RISC-V: Ignore V from the riscv,isa DT property on older T-Head CPUs

RISC-V: Ignore V from the riscv,isa DT property on older T-Head CPUs

Before attempting to support the pre-ratification version of vector
found on older T-Head CPUs, disallow "v" in riscv,isa on these
platforms. The deprecated property has no clear way to communicate
the specific version of vector that is supported and much of the vendor
provided software puts "v" in the isa string. riscv,isa-extensions
should be used instead. This should not be too much of a burden for
these systems, as the vendor shipped devicetrees and firmware do not
work with a mainline kernel and will require updating.

We can limit this restriction to only ignore v in riscv,isa on CPUs
that report T-Head's vendor ID and a zero marchid. Newer T-Head CPUs
that support the ratified version of vector should report non-zero
marchid, according to Guo Ren [1].

Link: https://lore.kernel.org/linux-riscv/CAJF2gTRy5eK73=d6s7CVy9m9pB8p4rAoMHM3cZFwzg=AuF7TDA@mail.gmail.com/
Fixes: dc6667a4e7e3 ("riscv: Extending cpufeature.c to detect V-extension")
Co-developed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20240223-tidings-shabby-607f086cb4d7@spud
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>