greybus: arche-platform: Rename cleanup fn to poweroff_seq

In sync with operational states, rename arche_platform_cleanup() fn
to arche_platform_poweroff_seq() fn.

Testing Done: Tested on EVT1.2 and DB3.5 platform.

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>