counter: stm32-lptimer-cnt: Provide defines for clock polarities

The STM32 low-power timer permits configuration of the clock polarity
via the LPTIMX_CFGR register CKPOL bits. This patch provides
preprocessor defines for the supported clock polarities.

Cc: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/a111c8905c467805ca530728f88189b59430f27e.1630031207.git.vilhelm.gray@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>