powerpc/pseries/rtas-work-area: Fix rtas_work_area_reserve_arena() kernel-doc

>From a W=1 build:

>> arch/powerpc/platforms/pseries/rtas-work-area.c:189: warning: Function parameter or member 'limit' not
>> described in 'rtas_work_area_reserve_arena'

Add the missing description of the limit parameter.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309131221.Bm1pg96n-lkp@intel.com/


Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231106-rtas-trivial-v1-1-61847655c51f@linux.ibm.com