Documentation: fix blackfin gptimers-example build errors

Don't build the blackfin gptimers-example module when BFIN_GPTIMERS
is not enabled. Allow the build when BFIN_GPTIMERS is =y or =m.

I believe that this patch fixes these build errors, but I don't
have a build environment to test this.

ERROR: "disable_gptimers" [Documentation/blackfin/gptimers-example.ko] undefined!
ERROR: "enable_gptimers" [Documentation/blackfin/gptimers-example.ko] undefined!
ERROR: "set_gptimer_config" [Documentation/blackfin/gptimers-example.ko] undefined!
ERROR: "clear_gptimer_intr" [Documentation/blackfin/gptimers-example.ko] undefined!
ERROR: "get_gptimer_period" [Documentation/blackfin/gptimers-example.ko] undefined!
ERROR: "get_gptimer_pwidth" [Documentation/blackfin/gptimers-example.ko] undefined!
ERROR: "get_gptimer_intr" [Documentation/blackfin/gptimers-example.ko] undefined!

Reported-by: Fenggaung Wu <fengguang.wu@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>