initcalls: Add initial implementation

Create simple serial subsystem which makes use of initcalls, and convert
existing serial drivers to its use.

drivers/pl011.c

@@ -18,17 +18,31 @@
     51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#include <init.h>
+#include <drivers/serial.h>
 #include <types.h>
 
 #define PL011_SERIAL_BASE 0x10009000
 #define PL011_SERIAL_FLAG_REGISTER 0x18
 #define PL011_SERIAL_BUFFER_FULL (1 << 5)
 
-void pl011_putc(char c)
+int pl011_putc(char c)
 {
 	/* Wait until the serial buffer is empty */
 	while (*(volatile uint32*)(PL011_SERIAL_BASE + PL011_SERIAL_FLAG_REGISTER) & (PL011_SERIAL_BUFFER_FULL));
 
 	/* When it's empty, put our character at the base */
 	*(volatile uint32*)PL011_SERIAL_BASE = c;
+
+	return 0;
 }
+
+struct serial_dev pl011_dev = {
+	.putc = &pl011_putc
+};
+
+void pl011_init() {
+	serial_register_device(&pl011_dev);
+}
+
+early_initcall(pl011_init);