avr: Split gpio.c into gpio.c, adc.c, hard_pwm.c, and spi.c

Split up gpio.c into multiple files in an effort to make the code a
little more understandable.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/avr/internal.h

@@ -0,0 +1,18 @@
+#ifndef __AVR_INTERNAL_H
+#define __AVR_INTERNAL_H
+// Local definitions for avr code
+
+#define GPIO(PORT, NUM) (((PORT)-'A') * 8 + (NUM))
+#define GPIO2PORT(PIN) ((PIN) / 8)
+#define GPIO2BIT(PIN) (1<<((PIN) % 8))
+
+struct gpio_digital_regs {
+    // gcc (pre v6) does better optimization when uint8_t are bitfields
+    volatile uint8_t in : 8, mode : 8, out : 8;
+};
+extern volatile uint8_t * const digital_regs[];
+
+#define GPIO2REGS(pin)                                                  \
+    ((struct gpio_digital_regs*)READP(digital_regs[GPIO2PORT(pin)]))
+
+#endif // internal.h