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>

include/asm-avr32/arch-at32ap700x/clk.h

@@ -84,7 +84,6 @@ static inline unsigned long get_spi_clk_rate(unsigned int dev_id)
 #endif
 
 extern void clk_init(void);
-extern void gclk_init(void) __attribute__((weak));
 
 /* Board code may need the SDRAM base clock as a compile-time constant */
 #define SDRAMC_BUS_HZ	(MAIN_CLK_RATE >> CFG_CLKDIV_HSB)