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>