arm: Adds board_postclk_init to the init_sequence.

The board_postclk_init() function can be used to perform operations
that requires a working timer early within the U-Boot init_sequence.

Signed-off-by: Markus Hubig <mhubig@imko.de>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>