stm32: lptim: add base support

Add basically what's needed to have some minimal but usefull subset of
function for a timer: irqs, compare, period, out polarity, enable/disable
and start.

lib/stm32/l4/Makefile

@@ -46,6 +46,7 @@ OBJS += flash.o flash_common_all.o flash_common_f.o flash_common_idcache.o
 OBJS += gpio_common_all.o gpio_common_f0234.o
 OBJS += i2c_common_v2.o
 OBJS += iwdg_common_all.o
+OBJS += lptimer_common_all.o
 OBJS += pwr.o
 OBJS += rcc.o rcc_common_all.o
 OBJS += rng_common_v1.o