clk: sirf: Silence sparse warnings

These are __iomem pointers. Mark them appropriately so we don't
get sparse errors like

drivers/clk/sirf/clk-common.c:60:16: warning: incorrect type in argument 1 (different address spaces)
drivers/clk/sirf/clk-common.c:60:16:    expected void const volatile [noderef] <asn:2>*addr
drivers/clk/sirf/clk-common.c:60:16:    got void *

Cc: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>