mx6cuboxi: Fix board revision detection

Currently, an i.MX6 Cuboxi board is incorrectly detected
as the HummingBoard model:

U-Boot 2024.04-rc5 (Mar 26 2024 - 15:59:22 +0100)

CPU:   Freescale i.MX6Q rev1.3 996 MHz (running at 792 MHz)
CPU:   Extended Commercial temperature grade (-20C to 105C) at 26C
Reset cause: POR
Model: SolidRun HummingBoard2 Dual/Quad (1.5som+emmc)
gpio@20a4000: set_dir_flags: error: gpio GPIO3_8 not reserved
gpio@20a4000: get_value: error: gpio GPIO3_8 not reserved
gpio@20a8000: set_dir_flags: error: gpio GPIO4_4 not reserved
gpio@20a8000: get_value: error: gpio GPIO4_4 not reserved
gpio@20b0000: set_dir_flags: error: gpio GPIO6_9 not reserved
gpio@20b0000: get_value: error: gpio GPIO6_9 not reserved
Board: MX6 HummingBoard
DRAM:  2 GiB
...

This error happens because request_detect_gpios() uses the GPIO DM
API, but board_type() still uses the legacy non-DM GPIO API.

Fix it by using the GPIO DM API in board_type() to read the
board revision pins in SPL.

Reported-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Christian Gmeiner <cgmeiner@igalia.com>