docs(xilinx): correct function description

Inside pm_ipi.c file, corrected the function description of
pm_ipi_buff_read_callb() and removed the return type as this is a void
function.

Signed-off-by: Naman Patel <naman.patel@amd.com>
Change-Id: I6257894337ef64497afb3e80d70af91a20357d5f
1 file changed