Blackfin: fix GPTMR0_CLOCKSOURCE dependency on BFIN_GPTIMERS

The GPTMR0_CLOCKSOURCE Kconfig option requires the gptimers framework, so
make sure it is selected when this option is enabled.

Reported-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>