clk: ingenic: Allow divider value to be divided

The JZ4780's MSC clock divider registers multiply the clock divider by 2.
This means that MMC devices run at half their expected speed. Add the
ability to divide the clock divider in order to solve this.

Signed-off-by: Harvey Hunt <harvey.hunt@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-clk@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>