rtc: ds1390: Fix the SPI chip select number

The number after the '@' symbol should be the SPI chip select
number associated with the ds1390 and it should match the 'reg'
field.

So change it the address to zero, to make it consistent.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>