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.

util_libs: remove autoconf.h from util_libs

this commit removes autoconf.h generation from each lib and
append '#include <<lib_name>/gen_config.h>' after each
'#include <autoconf.h>' since autoconf.h is only the for the
kernel config now. This is a temporarily solution, since we
currently don't have a way to tell which header files each file
is trying to include when it includes 'autoconf.h'

platsupport: Exynos: Update the mux_feature_enable and its invocations

Fix whitespace

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

fix licenses

Use CONFIG_ variables instead of configuration definitions

libplatsupport: Unify Chip select enum variants

Was introduced in pr #79. Previously these were defined per platform,
and when defining globally for TK1 there was a name conflict with
exynos.

Remove trailing whitespace

libplatsupport: use 180 degree phase feedback for SPI (Exynos)

This fixes issue #2

libplatsupport: Mark an argument to spi_prepare_transfer as `const`.

This avoids spurious warnings when passing in a pointer to a `const` struct.