dt-bindings: riscv: permit numbers in "riscv,isa"

There are some extensions that contain numbers, such as Zve32f, which
are enabled by the "max" cpu type in QEMU.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20231208-uncolored-oxidant-5ab37dd3ab84@spud


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>