board: rk3399: Drop explicit uart enablement in spl_board_init

preloader_console_init is used for printing SPL boot banner that
usually called from spl_board_init.

The current spl_board_init in evb and rock960 is enabling explicit
pinctrl, debug uart prior to calling preloader_console_init which
eventually not required since board_init_f is already enabled
debug uart.

So, drop those explicit enablement calls from  spl_board_init of
evb, rock960.

Tested this by enabling CONFIG_SPL_BOARD_INIT and adding
u-boot,dm-pre-reloc property for uart node.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>