ARM: OMAP: Catch callers of revision information prior to it being populated

ARM: OMAP: Catch callers of revision information prior to it being populated

omap_rev is used to detect various SoC types, however any misuse of
the usage by invoking it earlier than it being populated will result
in invalid results. Lets flag them as early as possible to prevent
unintended side effects taking place. We get 0 if it is uninitialized
and -1 when detection is done using device tree (as the case was for
DRA7 as the case was prior to commit d6f02a531ee9 ("ARM: OMAP: DRA7:
Make use of omap_revision information for soc_is* calls")

Signed-off-by: Nishanth Menon <nm@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>