dt-bindings: Update the riscv,isa string description

Since the RISC-V specification states that ISA description strings are
case-insensitive, there's no functional difference between mixed-case,
upper-case, and lower-case ISA strings. Thus, to simplify parsing,
specify that the letters present in "riscv,isa" must be all lowercase.
Suggested-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

Documentation/devicetree/bindings/riscv/cpus.yaml

@@ -50,6 +50,10 @@ properties:
       User-Level ISA document, available from
       https://riscv.org/specifications/
 
+      While the isa strings in ISA specification are case
+      insensitive, letters in the riscv,isa string must be all
+      lowercase to simplify parsing.
+
   timebase-frequency:
     type: integer
     minimum: 1