arduino nano: add cpu frequency to kconfig

src/arch/arduino-nano/Kconfig

 # Copyright 2020 Daniel Friesel
 #
 # SPDX-License-Identifier: CC0-1.0
+
+config arch_arduino_nano_cpufreq
+int "CPU Frequency"
+#!accept [62500, 125000, 250000, 500000, 1000000, 2000000, 4000000, 8000000, 16000000]
+range 62500 16000000
+default 16000000
+help
+  Assumes an externel 16MHz crystal to be present
+
 config arch_arduino_nano_driver_adc
 bool "ADC (Analog-Digital-Converter)"
 select meta_driver_adc

src/arch/arduino-nano/Makefile.inc

@@ -13,6 +13,10 @@ ifdef CONFIG_arch_arduino_nano_mega168
 	BAUD = 19200
 endif
 
+ifdef CONFIG_arch_arduino_nano_cpufreq
+	cpu_freq = ${CONFIG_arch_arduino_nano_cpufreq}
+endif
+
 cpu_freq ?= 16000000
 
 COMMON_FLAGS += -Werror=overflow