sam3: Merge sam4e8e support into sam3 code

Most of the peripherals on the sam4e8e are similar to the ones on the
sam3x8e mcu.  Merge the code together and use just one code directory.

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

src/sam3/spi.c

@@ -1,10 +1,9 @@
-// SPI transmissions on sam3x8e
+// SPI transmissions on sam3
 //
 // Copyright (C) 2018  Petri Honkala <cruwaller@gmail.com>
 //
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
-#include <sam3x8e.h> // REGPTR
 #include "command.h" // shutdown
 #include "gpio.h" // spi_setup
 #include "internal.h" // gpio_peripheral