realtek: enable pca953x driver for rtl838x subtarget

The system status LED on Panasonic Switch-M8eG PN28080K is connected to
a PCA9539PW. To use the LED as a status LED of OpenWrt while booting,
enable the pca953x driver and built-in to the kernel.
Also enable CONFIG_GPIO_PCA953X_IRQ to use interrupt via RTL83xx GPIO.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Acked-by: Sander Vanheule <sander@svanheule.net>