drivers/serial: stm32: Put LPUART code under LPUART Kconfig symbol
drivers/serial: stm32: Put LPUART code under LPUART Kconfig symbol

In order to ease integration of new series, remove reference to
series in code activation. Use LPUART support Kconfig symbol instead.

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

ldts/zephyr,kraj/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,ldts/zephyr,explora26/zephyr,kraj/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,ldts/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr