arm: mvebu: gdsys: Remove custom spl_board_init()

arm: mvebu: gdsys: Remove custom spl_board_init()

The gdsys a38x config file (controlcenterdc_defconfig) uses BootROM to
load U-Boot proper.

Since it is now possible to do this via U-Boot SPL framework, we do not
need to provide custom spl_board_init() which calls return_to_bootrom().

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>