UART pinctrl for serial is typically applied multiple times:
- in external TPL
- in SPL for DEBUG_UART in board_debug_uart_init()
- in SPL using pinctrl from DT
- in pre-reloc phase using pinctrl from DT
- after relocation using pinctrl from DT

Skip including UART pinctrl for the the pre-reloc phase to save time
during boot. On a Radxa ROCK 5B this reduce boot time by ~150 ms.

After this a warning may be shown with LOGLEVEL=5 (or higher), it is
safe to ignore this warning as device probe is successful even when
pinctrl cannot be applied:

  ns16550_serial serial@feb50000: pinctrl_select_state_full: uclass_get_device_by_phandle_id: err=-19

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>