riscv: Add basic support for SBI v0.2

riscv: Add basic support for SBI v0.2

The SBI v0.2 introduces a base extension which is backward compatible
with v0.1. Implement all helper functions and minimum required SBI
calls from v0.2 for now. All other base extension function will be
added later as per need.

As v0.2 calling convention is backward compatible with v0.1, remove
the v0.1 helper functions and just use v0.2 calling convention.

Add a new Kconfig options CONFIG_SBI for the new SBI v0.2 codes, and
let CONFIG_SBI_IPI depend on it.

This commit is inspired from Linux kernel patch:
https://patchwork.kernel.org/patch/11407363/

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>