sh: mach-sdk7786: Detect/configure/propagate EXTAL.

This uses the mode pins exposed through the FPGA to work out whether
we're driven from EXTAL or not and does the appropriate setup and
propagation through the clock framework.

This will also -EINVAL out for anyone adding in their own oscillators,
forcing proper configuration with the clock framework instead of
proceeding on with bogus clock values.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>