gpio: turris_omnia_mcu: Fix registering gpios

Currently all GPIOs supported by CMD_EXT_CONTROL/CMD_GET_EXT_CONTROL_STATUS
commands (last 16 GPIOs) are available only when FEAT_PERIPH_MCU feature
bit is set. So do not register these GPIOs by U-Boot driver when this
feature bit is not set, so U-Boot 'gpio' command would see only GPIOs which
really exists.

Fixes: 5e4d24cc ("gpio: Add Turris Omnia MCU driver")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>