ARM: omap5: Only select errata 798181 if SMP

Avoids the following warning when SMP is off:

warning: (ARCH_KEYSTONE && SOC_OMAP5) selects ARM_ERRATA_798181 which
has unmet direct dependencies (CPU_V7 && SMP)

Reported-by: Emilio López <emilio@elopez.com.ar>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Olof Johansson <olof@lixom.net>