ARM: OMAP2+: Fix omap4 errata warning on other SoCs

We have errata i688 workaround produce warnings on SoCs other than
omap4 and omap5:

omap4_sram_init:Unable to allocate sram needed to handle errata I688
omap4_sram_init:Unable to get sram pool needed to handle errata I688

This is happening because there is no ti,omap4-mpu node, or no SRAM
to configure for the other SoCs, so let's remove the warning based
on the SoC revision checks.

As nobody has complained it seems that the other SoC variants do not
need this workaround.

Signed-off-by: Tony Lindgren <tony@atomide.com>