iio:filter:admv8818: select REGMAP_SPI for ADMV8818

iio:filter:admv8818: select REGMAP_SPI for ADMV8818

admv8818 driver needs __devm_regmap_init_spi() which is defined
when CONFIG_REGMAP_SPI is set and struct regmap_config when
CONFIG_REGMAP is set, so automatically select CONFIG_REGMAP_SPI
which also sets CONFIG_REGMAP.

Fixes: f34fe888ad05 ("iio:filter:admv8818: add support for ADMV8818")
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220320055457.254983-1-bobo.shaobowang@huawei.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>