mfd: mfd-core: Complete kerneldoc header for devm_mfd_add_devices()

Each function parameter should be documented in kerneldoc format.

Squashes the following W=1 warnings:

 drivers/mfd/mfd-core.c:326: warning: Function parameter or member 'dev' not described in 'devm_mfd_add_devices'
 drivers/mfd/mfd-core.c:326: warning: Function parameter or member 'id' not described in 'devm_mfd_add_devices'
 drivers/mfd/mfd-core.c:326: warning: Function parameter or member 'cells' not described in 'devm_mfd_add_devices'
 drivers/mfd/mfd-core.c:326: warning: Function parameter or member 'n_devs' not described in 'devm_mfd_add_devices'
 drivers/mfd/mfd-core.c:326: warning: Function parameter or member 'mem_base' not described in 'devm_mfd_add_devices'
 drivers/mfd/mfd-core.c:326: warning: Function parameter or member 'irq_base' not described in 'devm_mfd_add_devices'
 drivers/mfd/mfd-core.c:326: warning: Function parameter or member 'domain' not described in 'devm_mfd_add_devices'

Signed-off-by: Lee Jones <lee.jones@linaro.org>