mfd: si476x: Add GPIOLIB_LEGACY dependency

This driver uses the legacy gpiolib interfaces to get gpio
numbers from platform data:

drivers/mfd/si476x-i2c.c: In function 'si476x_core_start':
drivers/mfd/si476x-i2c.c:133:21: error: implicit declaration of function 'gpio_is_valid'; did you mean 'uuid_is_valid'? [-Werror=implicit-function-declaration]
133 | if (gpio_is_valid(core->gpio_reset))

There are no in-tree users of this driver, so nothing defines
the platform data.

Add a dependency on GPIOLIB_LEGACY for the moment to avoid the build
failure, and make sure the sound driver does not get built without the
mfd portion either pass that dependency along.

Alternatively, we could remove the mfd driver along with the radio and
sound portions.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202507231653.UFlH2dMO-lkp@intel.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20250808151822.536879-14-arnd@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>