dt-bindings: riscv: drop invalid comment about riscv,isa lower-case reasoning

"Ease of parsing" may have been the initial argument for keeping this
string in lower-case, but parsers may have been written that expect
lower-case only.
For example, the one in released kernels currently does not behave
correctly for multi-letter extensions that begin with a capital letter.
Allowing upper-case here brings about no benefit but would break
compatibility between new devicetrees and older kernels.

Drop the comment to avoid confusing people.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Yangyu Chen <cyy@cyyself.name>
Link: https://lore.kernel.org/r/tencent_3B8290DDC66D3E624132ED39C7465CDC9807@qq.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -94,7 +94,7 @@ properties:
 
       While the isa strings in ISA specification are case
       insensitive, letters in the riscv,isa string must be all
-      lowercase to simplify parsing.
+      lowercase.
     $ref: "/schemas/types.yaml#/definitions/string"
     pattern: ^rv(?:64|32)imaf?d?q?c?b?k?j?p?v?h?(?:[hsxz](?:[a-z])+)?(?:_[hsxz](?:[a-z])+)*$