lib: sbi: Implement System Reset (SRST) SBI extension

The SBI SRST extension has been accepted and merged in the latest
SBI v0.3-draft specification.
(Refer, https://github.com/riscv/riscv-sbi-doc

)

It allows to S-mode software to request system shutdown, cold reboot,
and warm reboot.

This patch implements SBI SRST extension as a replacement of the
legacy sbi_shutdown() call of SBI v0.1 specification.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>