gpio: mlxbf: Add dependency on 64BIT to Kconfig entry

Fixes a compile test failure

Fixes: c78c42d7

 ("gpio: add driver for Mellanox BlueField GPIO controller")
Signed-off-by: Shravan Kumar Ramani <sramani@mellanox.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>