avr32: Use board_postclk_init instead of gclk_init

Replace the avr32-specific gclk_init() board hook with the standard
board_postclk_init() hook which is supposed to run at the same point
during initialization.

Provide a dummy weak alias for boards not implementing this hook. The
cost of this is:
  - 2 bytes for the dummy function (retal 0)
  - 2 bytes for each unnecessary function call (short rcall)

which is a pretty small price to pay for avoiding lots of #ifdef
clutter. In this particular case, all boards probably end up slightly
smaller because we avoid the conditional checking if the gclk_init
symbol is NULL.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
diff --git a/board/miromico/hammerhead/hammerhead.c b/board/miromico/hammerhead/hammerhead.c
index bf432cb..8670527 100644
--- a/board/miromico/hammerhead/hammerhead.c
+++ b/board/miromico/hammerhead/hammerhead.c
@@ -100,9 +100,10 @@
 	gd->bd->bi_phy_id[0] = 0x01;
 }
 
-void gclk_init(void)
+int board_postclk_init(void)
 {
 	/* Hammerhead boards uses GCLK3 as 25MHz output to ethernet PHY */
 	gclk_enable_output(3, PORTMUX_DRIVE_LOW);
 	gclk_set_rate(3, GCLK_PARENT_OSC0, 25000000);
+	return 0;
 }