riscv: Rework and cleanup Kconfig

This patch is doing several things:

- Core ISA and extension Kconfig symbols have now a formalized name
  (CONFIG_RISCV_ISA_* and CONFIG_RISCV_ISA_EXT_*)

- a new Kconfig.isa file was introduced with the full set of extensions
  currently supported by the v2.2 spec

- a new Kconfig.core file was introduced to host all the RISCV cores
  (currently only E31)

- ISA and extensions settings are moved to SoC configuration files

Signed-off-by: Carlo Caione <ccaione@baylibre.com>