ARM: OMAP2+: split omap2/3/4_check_revision function

We need to detect the SoC revision early, but the SoC
feature detection can be done later on. In order to allow
further clean-up later on, this patch separates the SoC
revision check from the SoC feature check.

This patch doesn't change functionality or behavior of the code
execution; it barely cleans up the code and splits into SoC
specific implementation for Rev ID and feature detection.

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