firmware: ti_sci: Update ti_sci_msg_req_reboot to include domain

The ti_sci_msg_req_reboot message payload has been extended to include a
domain field, but for the purposes of u-boot this should be zero to
reset the entire SoC as it did before. Include domain for completeness
and set to zero to ensure proper operation.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>