MIPS: AR7: allow NULL clock for clk_get_rate

MIPS: AR7: allow NULL clock for clk_get_rate

Make the behaviour of clk_get_rate consistent with common clk's
clk_get_rate by accepting NULL clocks as parameter. Some device
drivers rely on this, and will cause an OOPS otherwise.

Fixes: 780019ddf02f ("MIPS: AR7: Implement clock API")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reported-by: Mathias Kresin <dev@kresin.me>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16775/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>