board: phytec: phycore-am62x: Fix CONFIG_SPL_BOARD_INIT

Make sure spl_board_init() gets compiled by enabling missing
CONFIG_SPL_BOARD_INIT and including hardware.h.

Fixes: 085cd645 ("board: phytec: am62x: Add PHYTEC phyCORE-AM62x SoM")

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Reviewed-by: Daniel Schultz <d.schultz@phytec.de>