Add a sentinel to ab85xx_rtc_ids[] in order to fix the following error:

drivers/rtc/rtc-ab8500: struct platform_device_id is 24 bytes.  The last of 2 is:
0x61 0x62 0x38 0x35 0x34 0x30 0x2d 0x72 0x74 0x63 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x8c
FATAL: drivers/rtc/rtc-ab8500: struct platform_device_id is not terminated with a NULL entry!
Reported-by: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Reported-by: Olof's autobuilder <build@lixom.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>