ARM: OMAP4: remove dead kconfig option OMAP4_ERRATA_I688

The Kconfig-Option OMAP4_ERRATA_I688 is never visible due to a
contradiction in it's dependencies.
The option requires ARCH_MULTIPLATFORM to be 'disabled'. However, an
enclosing menu requires either ARCH_MULTI_V6 or ARCH_MULTI_V7 to be
enabled. These options inherit a dependency from an enclosing menu,
that requires ARCH_MULTIPLATFORM to be 'enabled'.
This is a contradiction and made this option also unavailable for
non-multiplatform configurations.

Since there are no selects on OMAP4_ERRATA_I688, which would ignore
dependencies, the code related to that option is dead and can be
removed.

This (logical) defect has been found with the undertaker tool.
(https://undertaker.cs.fau.de

)

Signed-off-by: Stefan Hengelein <stefan.hengelein@fau.de>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>