OMAP2+: mux: Fixed gpio mux mode analysis

OMAP_MODE_GPIO() macro verified only OMAP_MUX_MODE4.
It is not correct for following platforms:
    2430 - gpio mux mode 3
    44xx - gpio mux mode 3
    54xx - gpio mux mode 6

Patch reserves first 3 bits in partition flags for storing gpio mux
mode in same format as stored in control pad register.
Modified OMAP_MODE_GPIO() macro to handle all possible cases of gpio mux mode.
Modified omap_mux_init() flags of omap34xx to include OMAP_MUX_GPIO_IN_MODE4.

Signed-off-by: Oleg Matcovschi <oleg.matcovschi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>