clk: imx: add configuration option for mmio clks

The patch introduces CONFIG_MXC_CLK option for legacy MMIO clocks,
this is required to compile legacy MMIO clock conditionally when adding
SCU based clocks for MX8 platforms later.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>