rockchip: rk3399: Fix TPL build of bob and kevin

rockchip: rk3399: Fix TPL build of bob and kevin

Building chromebook_bob/kevin with TPL=y ends with a linking error:

  arch/arm/mach-rockchip/rk3399/rk3399.o: in function `board_debug_uart_init':
  arch/arm/mach-rockchip/rk3399/rk3399.c:148:(.text.board_debug_uart_init+0x34):
    undefined reference to `spl_gpio_output'
  arch/arm/mach-rockchip/rk3399/rk3399.c:148:(.text.board_debug_uart_init+0x34):
    relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `spl_gpio_output'

  make[2]: *** [scripts/Makefile.xpl:542: tpl/u-boot-tpl] Error 1
  make[1]: *** [Makefile:2134: tpl/u-boot-tpl] Error 2
  make: *** [Makefile:568: __build_one_by_one] Error 2

Change to only use spl_gpio functions in SPL to fix this.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>