iio: adc: Add driver for Renesas RZ/G2L A/D converter

Add ADC driver support for Renesas RZ/G2L A/D converter in SW
trigger mode.

A/D Converter block is a successive approximation analog-to-digital
converter with a 12-bit accuracy and supports a maximum of 8 input
channels.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Link: https://lore.kernel.org/r/20210804202118.25745-3-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>