ARM: SIRF: select SMP_ON_UP only on SMP builds

Like all other platforms, we can only select SMP_ON_UP if SMP
is also enabled.

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

Cc: Barry Song <baohua.song@csr.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>