ARM: keystone: select ARM_ERRATA_798181 only for SMP

Selecting this symbol causes a build warning without SMP:

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

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>