arch/nrf91: modem depends on LFCLK

arch/arm/src/nrf91/Kconfig

@@ -669,6 +669,7 @@ config NRF91_MODEM
 	depends on ALLOW_BSDNORDIC_COMPONENTS && ARCH_TRUSTZONE_NONSECURE
 	select ARCH_IRQPRIO
 	select NRF91_IPC
+	select NRF91_USE_LFCLK
 	default n
 
 if NRF91_MODEM

arch/arm/src/nrf91/nrf91_modem.c

@@ -43,6 +43,10 @@
 #  error NRF91 modem works only in non-secure environment
 #endif
 
+#ifndef CONFIG_NRF91_LFCLK_XTAL
+#  error NRF91 modem requires using LFXO as the LFCLK source
+#endif
+
 /****************************************************************************
  * Private Functions
  ****************************************************************************/