dts/riscv: add missing `riscv,isa` fields and modify existing ones

This commit adds/modifies `riscv,isa` strings using the following rules:
* the ISA string is lowercase
* multi-letter extensions are preceded with the underscore mark
* if an extension is implied by another one, it is not specified - e.g. the
  D extension implies the F extension, so writing `rv32ifd` is redundant

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>