Even though the tfa9879 driver can probe via device tree trough the
I2C core code, it is preferable to have explicit device tree
bindings instead [1], so add this support.

[1] https://www.spinics.net/lists/devicetree/msg195176.htmlSigned-off-by: NFabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: NŁukasz Majewski <lukma@denx.de>
Signed-off-by: NMark Brown <broonie@kernel.org>