drivers: serial: gecko: add hw flow control support for EFR32 Series 2

Add hardware flow control support for the EFR32MG21 and other modules
using GPIO_USART_ROUTEEN_RTSPEN (and GPIO_USART_ROUTEEN_CTSEN) instead
of the Kconfig SOC_GECKO_HAS_INDIVIDUAL_PIN_LOCATION.

The driver already contained the section settings the RTS and CTS pins
for modules using the GPIO_USART_ROUTEEN_(RTS|CTS)PEN define, but it was
not compiling because of an #ifdef checking only for
CONFIG_SOC_GECKO_HAS_INDIVIDUAL_PIN_LOCATION and not both.

Signed-off-by: Steven Lemaire <steven.lemaire@zii.aero>