serial: OMAP2+: UART: Make the SERIAL_OMAP depend on ARCH_OMAP2PLUS

Making  SERIAL_OMAP depend on ARCH_OMAP2PLUS instead of
oring with ARCH2/3/4.

Acked-by: Felipe Balbi <balbi@ti.com>
Suggested-by: Sricharan R <r.sricharan@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>