mhi: core: add MHI ready state transition support

MHI state transition ready was not handled before, adding support
for MHI ready state.

CRs-Fixed: 2257733
Change-Id: Ib31f353f5a0ca6d160c94b0604d4d836281f58ad
Signed-off-by: Gustavo Solaira <gustavos@codeaurora.org>
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>