arm: mvebu: Convert board_pex_config() to CONFIG_SPL_BOARD_INIT

arm: mvebu: Convert board_pex_config() to CONFIG_SPL_BOARD_INIT

The only user of board_pex_config() weak function is A385 controlcenterdc
board. It looks like that code in its board_pex_config() function needs to
be executed after PCIe link is up. Therefore put this code into
spl_board_init() function which is called after a38x serdes initialization,
and therefore it is after the serdes hws_pex_config() function finishes
(which is the state before this change).

With this change completely remove board_pex_config() function as it is not
used anymore.

Signed-off-by: Pali Rohár <pali@kernel.org>