soc/x86: Clean up EHL kconfigs

Cleanup along the same lines as the last change to APL/up_squared.
Make sure all hardware configuration is at the board level where it
belongs and not in the soc, don't play games with defaulting timer
drivers.  Unify the configuration where possible and make it clearer
which setting goes with which driver.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>