platsupport: Don't require plat/mux.h on all plats

As mux_sys_init is currently required on all Arm platforms, having a
enum definition for all mux features that a platform has requires
defining the enum on all platforms.
We instead replace enum mux_feature with a typedef'd ssize_t and allow
platforms to define enum mux_feature definitions at an implementation
level rather than interface level.

platsupport: mux: Fix build errors for platforms that haven't been directly targeted

platsupport: Mux API: Add a dependencies parameter to mux_sys_init

This is a workaround for the fact that we have no real dependency tree at the
moment. The two approaches to this problem were either:
* Add a gpio_sys_t member inside of ps_io_ops_t, similar to the way we already have
a clock_sys_t and a mux_sys_t in there.
* Add a void *dependencies argument to the mux_init function that will enable us to
pass arbitrary extra dependencies in.

The latter was chosen because while it is more intrusive, it is slightly more
flexible.

Fix whitespace

- remove trailing whitespace
- remove duplicate blank lines
- remove blank lines at end of file

fix licenses