platform: cznic: turris-omnia-mcu: Rename variable holding GPIO line names

Rename the `omnia_mcu_gpio_templates` variable to
`omnia_mcu_gpio_names`. The array contained templates for the names
during the development of the driver, but the template prefix `gpio%u.`
was dropped before the driver was merged, since this functionality was
broken in gpiolib.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>