ARM: imx: Enable SPL_BOARD_INIT on DH i.MX8M Plus DHCOM

The CONFIG_SPL_BOARD_INIT lets SPL common code call spl_board_init()
during the SPL start up. On this particular system, spl_board_init()
is used to reconfigure GIC clock parent to PLL2 500M, which is the
configuration expected by the Linux kernel. Enable SPL_BOARD_INIT .

Set GIC clock to 500 MHz for OD VDD_SOC. Kernel driver does not
allow to change it. Should set the clock after PMIC setting done.
Default is 400 MHz (system_pll1_800m with div = 2) set by ROM for
ND VDD_SOC.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>