riscv: add Kconfig entries for the C and A ISA extensions

Add Kconfig entries for the C (compressed instructions) and A (atomic
instructions) ISA extensions. Only the C ISA extension is selectable.
This matches the configuration in Linux.

The Kconfig entries are not used yet. A follow-up patch will select the
appropriate compiler flags based on the Kconfig configuration.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>