omap: Fix functions for dynamic remuxing of pins

Make the omap_mux_read and write available for board code,
and rename omap_mux_set_board_signals into omap_mux_write_array.
Also add the related prototypes and comments into mux.h.

In some cases we want to change the signals dynamically,
mostly for power management.

Note that we cannot use the signal names as they are set
__init to save memory.

Signed-off-by: Tony Lindgren <tony@atomide.com>