mvebu: fix boot stalls on WRT1900AC V1

Signed-off-by: John Crispin <[email protected]>

target/linux/mvebu/config-4.4

@@ -209,6 +209,7 @@ CONFIG_HAVE_UID16=y
 CONFIG_HAVE_VIRT_CPU_ACCOUNTING_GEN=y
 CONFIG_HIGHMEM=y
 # CONFIG_HIGHPTE is not set
+CONFIG_HOTPLUG_CPU=y
 CONFIG_HWBM=y
 CONFIG_HWMON=y
 CONFIG_HZ_FIXED=0