dt-bindings: riscv: fix underscore requirement for multi-letter extensions

The RISC-V ISA Manual allows the first multi-letter extension to avoid
a leading underscore. Underscores are only required between multi-letter
extensions.

The dt-binding does not validate that a multi-letter extension is
canonically ordered, as that'd need an even worse regex than is here,
but it should not fail validation for valid ISA strings.

Allow the first multi-letter extension to appear immediately after
the single-letter extensions.

Link: https://github.com/riscv/riscv-isa-manual/releases/tag/riscv-unpriv-pdf-from-asciidoc-15112022 # Chapter 29.5
Fixes: 299824e68bd0 ("dt-bindings: riscv: add new riscv,isa strings for emulators")
Acked-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20221205174459.60195-2-conor@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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