pinmux: remove user mode access

The pinmux subsystem does not have any API documenation. Currently
none of the existing pinmux drivers do any kind of input validation
for the pin/op parameters, which has been shown to allow the caller
to access unauthorized memory, up to and including disabling the
MPU.

This requires a proper specification for the pinmux subsystem and
also modification of existingd drivers to validate arguments on a per
driver basis. Remove user mode access until #7390 is resolved.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>