rockchip: spl: Drop useless call to debug_uart_init

Since commit 0dba4586 ("arm: Init the debug UART") function
debug_uart_init is called in crt files _main before calling
board_init_f. Therefore, there is no need to call it again
inside board_init_f implementation in arm/mach-rockchip/spl.c.

Signed-off-by: Massimo Pegorer <massimo.pegorer+oss@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>