cmd/riscv/sbi: support System Reset Extension

Let the sbi command detect the 'System Reset Extension'
(EID #0x53525354 "SRST").

Cf. https://github.com/riscv/riscv-sbi-doc



Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Pragnesh Patel <pragnesh.patel@openfive.com>
Reviewed-by: Leo Liang <ycliang@andestech.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>