clk: mvebu: fix spelling mistake "gatable" -> "gateable"

There are a few spelling mistakes in comments and a pr_err
error message. Fix these.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>