doc: nvmem: Remove references to regmap

Since commit 795ddd18

 ("nvmem: core: remove regmap dependency"),
nvmem devices do not use the regmap API. Remove references to it from
the documentation.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220220151527.17216-11-srinivas.kandagatla@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>