dts: bindings: riscv: Add sifive-e24 cpu

Add sifive-e24 cpu binding.  This introduce riscv,cpu binding to
be used as riscv cpu base and riscv,sifive, which define specific
properties for this vendor.  Both are necessary to create the e24
core.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>