clk: mvebu: Mark ioremapped memory as __iomem

Silence the following sparse warning

drivers/clk/mvebu/dove-divider.c:252:14: warning: incorrect type in assignment (different address spaces)
drivers/clk/mvebu/dove-divider.c:252:14:    expected void *base
drivers/clk/mvebu/dove-divider.c:252:14:    got void [noderef] <asn:2>*
drivers/clk/mvebu/dove-divider.c:256:13: warning: incorrect type in argument 2 (different address spaces)
drivers/clk/mvebu/dove-divider.c:256:13:    expected void [noderef] <asn:2>*base
drivers/clk/mvebu/dove-divider.c:256:13:    got void *base
drivers/clk/mvebu/dove-divider.c:257:25: warning: incorrect type in argument 1 (different address spaces)
drivers/clk/mvebu/dove-divider.c:257:25:    expected void volatile [noderef] <asn:2>*iomem_cookie
drivers/clk/mvebu/dove-divider.c:257:25:    got void *base

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>