common: board_f: Do not call board_postclk_init twice

The generic-board board_init_f function called board_postclk_init twice.

The first one came from arch/arm/lib/board.c, while the second one
from arch/powerpc/lib/board.c.

This commit deletes the first occurrence.
In addition, the second get_clocks call is moved after
board_postclk_init in order to keep the function call order
both for ARM and PowerPC.
ARM board calles get_clocks function after board_postclk_init.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>