This adds support for the i.MX8MM.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

The i.MX8M boards all have the same code for setting up the UART clock.
Add a common helper for it. In the helper just setup the clocks for all
UARTs as it's not worth it to have separate functions for each UART.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

U-Boot has some lowlevel clock functions which take a clock slice index
as argument. Add them for barebox as well to make the code better
comparable to U-Boot.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

i.MX8 is something different than the i.MX8M and both will not share
this header file, so rename it to imx8m-ccm-regs.h

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>