include: sbi_platform: Combine reboot and shutdown into one callback

We can achieve shutdown, cold reboot, and warm reboot using just
one sbi_platform callback so we combine system_reboot() and
system_shutdown() callbacks into one system_reset() callback.

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