Blackfin: update gptimers API

First we fix the prototypes for functions that return boolean values by
using "int" rather than "uint16_t".  Then we introduce a get_gptimer_run()
function for checking the current run status of a timer, and then we add a
disable_gptimers_sync() function which parallels disable_gptimers() with
corresponding normal "_sync" behavior.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>