ARM: keystone2: Switch to using the poweroff command

Now that we have a standard way to power off the hardware, switch to
using that rather than our own command.

Cc: Vitaly Andrianov <vitalya@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>