clk: iproc: Add PLL base write function

All writes to the PLL base address must be flushed if the
IPROC_CLK_NEEDS_READ_BACK flag is set.  If we add a function to make the
necessary write and reads, we can make sure that any future code which
makes PLL base writes will do the correct thing.

Signed-off-by: Jon Mason <jonmason@broadcom.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>