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

arch/arm/mach-omap2/Kconfig

@@ -62,7 +62,7 @@ config SOC_OMAP5
 	select HAVE_SMP
 	select COMMON_CLK
 	select HAVE_ARM_ARCH_TIMER
-	select ARM_ERRATA_798181
+	select ARM_ERRATA_798181 if SMP
 
 config SOC_AM33XX
 	bool "AM33XX support"