iio: health/afe4403: select REGMAP_SPI

The newly added afe4403 driver uses the regmap facility to abstract
the I2C and SPI access. However, it fails to ensure that regmap_spi
is actually present:

drivers/iio/built-in.o: In function `afe4403_probe':
:(.text+0x9bf8): undefined reference to `__devm_regmap_init_spi'

This adds a Kconfig select statement like the afe4404 I2C driver
has.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: eec96d1e ("iio: health: Add driver for the TI AFE4403 heart monitor")
Acked-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>