mfd: altera-sysmgr: Supply descriptions for 'np' and 'property' function args

Kerneldoc syntax is used, but not complete.  Arg descriptions are required.

Fixes the following W=1 build warnings:

 drivers/mfd/altera-sysmgr.c:95: warning: Function parameter or member 'np' not described in 'altr_sysmgr_regmap_lookup_by_phandle'
 drivers/mfd/altera-sysmgr.c:95: warning: Function parameter or member 'property' not described in 'altr_sysmgr_regmap_lookup_by_phandle'

Cc: Thor Thayer <thor.thayer@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>