Save 16 bytes RAM by placing esp8266_gpioToFn (core_esp8266_wiring_digital.cpp) array in PROGMEM (#6703)

* Save 16 bytes RAM by placing the array in PROGMEM.

* Reduce runtime computation at expense of switching from uint8_t[16] to uint32_t*[16]

Author: dok-net

cores/esp8266/core_esp8266_wiring_digital.cpp

@@ -30,7 +30,7 @@
 
 extern "C" {
 
-uint8_t esp8266_gpioToFn[16] = {0x34, 0x18, 0x38, 0x14, 0x3C, 0x40, 0x1C, 0x20, 0x24, 0x28, 0x2C, 0x30, 0x04, 0x08, 0x0C, 0x10};
+volatile uint32_t* const esp8266_gpioToFn[16] PROGMEM = { &GPF0, &GPF1, &GPF2, &GPF3, &GPF4, &GPF5, &GPF6, &GPF7, &GPF8, &GPF9, &GPF10, &GPF11, &GPF12, &GPF13, &GPF14, &GPF15 };
 
 extern void __pinMode(uint8_t pin, uint8_t mode) {
   if(pin < 16){

cores/esp8266/esp8266_peri.h

@@ -102,8 +102,8 @@
 #define GPF14  ESP8266_REG(0x80C)
 #define GPF15  ESP8266_REG(0x810)
 
-extern uint8_t esp8266_gpioToFn[16];
-#define GPF(p) ESP8266_REG(0x800 + esp8266_gpioToFn[(p & 0xF)])
+extern volatile uint32_t* const esp8266_gpioToFn[16];
+#define GPF(p) (*esp8266_gpioToFn[(p & 0xF)])
 
 //GPIO (0-15) PIN Function Bits
 #define GPFSOE 0 //Sleep OE