frv: fix compiler warning from definition of __pmd()

Take into account that the pmd_t type is a array inside a struct, so it
needs two levels of brackets to initialize.  Otherwise, a usage of __pmd
generates a warning:

  include/linux/mm.h:986:2: warning: missing braces around initializer [-Wmissing-braces]

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>