nvmem: update Kconfig description

nvmem can't be built as module anymore, update its Kconfig description.

Fixes: 2a37ce25 ("nvmem: disallow modular CONFIG_NVMEM")
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200722100705.7772-13-srinivas.kandagatla@linaro.org


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