ARM: OMAP2+: am33xx: Change cpu_is_am33xx to soc_is_am33xx

As per recent discussion on the linux-omap list, we are
moving in the direction where, we will have only architecture,
ARCH_OMAP2PLUS and all devices/platforms will be treated
as a SoC underneath.

So the first step in this direction is to adopt this change
for all new devices getting in, converting
cpu_is_am33xx/335x() ==> soc_is_am33xx/335x()

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>