ARM: realview: don't select SMP_ON_UP for UP builds

When CONFIG_SMP is disabled, we get a warning from Kconfig:

warning: (SOC_IMX31 && SOC_IMX35 && SOC_VF610 && REALVIEW_DT) selects SMP_ON_UP which has unmet direct dependencies (SMP && !XIP_KERNEL && MMU)

This changes the REALVIEW_DT Kconfig entry to not select SMP_ON_UP
unless SMP is also set.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>