nvmem: core: describe add missing dev function parameter

Document dev parameter which not described in devm_nvmem_unregister
and devm_nvmem_register functions.

Fix below warnings when kernel is compiled with W=1
drivers/nvmem/core.c:579: warning: Function parameter or member
 'dev' not described in 'devm_nvmem_register'
nvmem/core.c:615: warning: Function parameter or member 'dev'
 not described in 'devm_nvmem_unregister'

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>