greybus: timesync: Implement a retry mechanism

It's possible the AP could miss an incoming SVC timesync pulse i.e. the AP
could have interrupts switched off for long enough that one SVC GPIO strobe
ends up over-lapping another one. TimeSync should be able to deal with this
type of transitory failure by retrying a failed synchronous TimeSync resync
up to 'n' number of times. For this patch 'n' has been set to five, which
is a hand-wavy choice that 'feels' right.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>