arm-bsp-extras/oeqa: Improvements to SCMI tests

Power-off and reboot tests have been extended to account
for different Linux kernel versions, i.e. whether if it
is linux-yocto-rt or linux-yocto.

The reboot test has further been extended to include
checkpoints for the login prompt before logging in
to Linux. Timeouts have also been extended.

Signed-off-by: Mubashar Ahmad <mubashar.ahmad@arm.com>