Blackfin arch: Typo: "CONFIG_RTC_BFIN_MODULE" -> "CONFIG_RTC_DRV_BFIN_MODULE"

i'm *reasonably* confident that this is a typo that should be fixed.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>