clk: imx: imx21: Remove clock driver

As i.MX21 support has been removed, get rid of its clock driver too.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>