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>