Add portInputRegister API

Very legacy but still in use (by DHT library, for example).
Need to test if digitalPinToPort works correctly too with legacy libraries declaing special sections guarded by #ifdef __AVR__

Author: facchinm

cores/arduino/Arduino.h

@@ -124,6 +124,8 @@ extern const uint8_t PROGMEM digital_pin_to_interrupt[];
 #define analogPinToBitMask(pin) ( (pin < NUM_ANALOG_INPUTS) ? (1 << analogPinToBitPosition(pin)) : NOT_A_PIN )
 #define digitalPinToTimer(pin) ( (pin < NUM_TOTAL_PINS) ? pgm_read_byte(digital_pin_to_timer + pin) : NOT_ON_TIMER )
 
+#define portInputRegister(port) ( &(((PORT_t*)(&PORTA + port))->IN) )
+
 #define portToPortStruct(port) ( (port < NUM_TOTAL_PORTS) ? ((PORT_t *)&PORTA + port) : NULL)
 #define digitalPinToPortStruct(pin) ( (pin < NUM_TOTAL_PINS) ? ((PORT_t *)&PORTA + digitalPinToPort(pin)) : NULL)
 #define getPINnCTRLregister(port, bit_pos) ( ((port != NULL) && (bit_pos < NOT_A_PIN)) ? ((uint8_t *)&(port->PIN0CTRL) + bit_pos) : NULL )