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: 299824e6 ("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.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -80,7 +80,7 @@ properties:
       insensitive, letters in the riscv,isa string must be all
       lowercase to simplify parsing.
     $ref: "/schemas/types.yaml#/definitions/string"
-    pattern: ^rv(?:64|32)imaf?d?q?c?b?v?k?h?(?:_[hsxz](?:[a-z])+)*$
+    pattern: ^rv(?:64|32)imaf?d?q?c?b?v?k?h?(?:[hsxz](?:[a-z])+)?(?:_[hsxz](?:[a-z])+)*$
 
   # RISC-V requires 'timebase-frequency' in /cpus, so disallow it here
   timebase-frequency: false