iio: accel: st_accel: Add support for Silan SC7A20

iio: accel: st_accel: Add support for Silan SC7A20

This chip appears to be register-compatible with the LIS2DH. The new
description is a copy of the LIS2DH's description with a different WAI
value.

Datasheet: http://linux-chenxing.org/silan/SC7A20-SilanMicroelectronics.pdf
Datasheet: http://www.siitek.com.cn/Upfiles/down/SC7A20说明书_0.92(智能穿戴).pdf
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220202235049.8051-4-samuel@sholland.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>