Add reg::io_addr() and AVR_IO_SFR_OFFSET

+ Add a method to return the register address inside the I/O space.
+ Define a macro for each supported MCU to indicate the memory offset to
map an I/O register into data memory space.

include/avr/io/mcu/atmega328p.hpp

@@ -1,4 +1,6 @@
 #pragma once
 
+#define AVR_IO_SFR_OFFSET 0x20
+
 #include "avr/io/mcu/atmega328p/pins.hpp"
 #include "avr/io/mcu/atmega328p/regs.hpp"

include/avr/io/mcu/attiny13a.hpp

@@ -1,4 +1,6 @@
 #pragma once
 
+#define AVR_IO_SFR_OFFSET 0x20
+
 #include "avr/io/mcu/attiny13a/pins.hpp"
 #include "avr/io/mcu/attiny13a/regs.hpp"

include/avr/io/mcu/attiny85.hpp

@@ -1,4 +1,6 @@
 #pragma once
 
+#define AVR_IO_SFR_OFFSET 0x20
+
 #include "avr/io/mcu/attiny85/pins.hpp"
 #include "avr/io/mcu/attiny85/regs.hpp"

include/avr/io/register/reg.hpp

@@ -20,6 +20,14 @@ struct reg : detail::reg_base {
     /** Memory address of the register. */
     static constexpr uint8_t addr = address;
 
+    /** Return the register address inside the I/O space. */
+    static constexpr uint8_t io_addr() {
+      static_assert(addr < 0x60,
+        "data memory address greater than or equal to 0x60 doesn't correspond" \
+        " to a register with an address on I/O space.");
+      return addr - AVR_IO_SFR_OFFSET;
+    }
+
     /** Returns a reference to read/write. */
     static volatile uint8_t& ref() noexcept
     { return *reinterpret_cast<volatile uint8_t*>(addr); }