ar71xx: mach-rbspi: fix 74x164 support

The platform data was missing the num_registers element which is now
mandatory in linux 4.9

Without this patch, the gpio probing would fail with:
gpio gpiochip1: (74x164): tried to insert a GPIO chip with zero lines

Fixes: #1106

Tested-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>

target/linux/ar71xx/files/arch/mips/ath79/mach-rbspi.c

@@ -488,6 +488,7 @@ static struct gpio_keys_button rblhg_gpio_keys[] __initdata = {
 
 static struct gen_74x164_chip_platform_data rbspi_ssr_data = {
 	.base = RBSPI_SSR_GPIO_BASE,
+	.num_registers = 1,
 };
 
 /* the spi-ath79 driver can only natively handle CS0. Other CS are bit-banged */