feat(ti): add enter sleep method

This TISCI API must be used to trigger entry into system suspend, and
this is done through the use of TI_SCI_MSG_ENTER_SLEEP. Introduce a
method to send this message.

Change-Id: Id7af5fb2a34623ad69e76764f389ff4d8d259fba
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
3 files changed