powerpc/eeh_cache: fix a W=1 kernel-doc warning

The opening comment mark "/**" is reserved for kernel-doc comments, so
it will generate a warning with "make W=1".

arch/powerpc/kernel/eeh_cache.c:37: warning: cannot understand function
prototype: 'struct pci_io_addr_range

Since this is not a kernel-doc for the struct below, but rather an
overview of this source eeh_cache.c, just use the free-form comments
kernel-doc syntax instead.
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/eeh_cache.c

@@ -31,6 +31,8 @@
 
 
 /**
+ * DOC: Overview
+ *
  * The pci address cache subsystem.  This subsystem places
  * PCI device address resources into a red-black tree, sorted
  * according to the address range, so that given only an i/o
@@ -47,6 +49,7 @@
  * than any hash algo I could think of for this problem, even
  * with the penalty of slow pointer chases for d-cache misses).
  */
+
 struct pci_io_addr_range {
 	struct rb_node rb_node;
 	resource_size_t addr_lo;