riscv: Implement new SBI v0.2 extensions

Few v0.1 SBI calls are being replaced by new SBI calls that follows
v0.2 calling convention.

Implement the replacement extensions and few additional new SBI
function calls that makes way for a better SBI interface in future.

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