clk: h8300: Properly cast to __iomem pointer

Sparse complains here because we dropped the __iomem annotation
when casting the aligned address. Add __iomem back so that sparse
stops complaining.

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: <uclinux-h8-devel@lists.sourceforge.jp>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>