[rpi3] Move SOC_BRCM_BCM2837 from UP config to SMP one

Now that BCM283x source are buildable with SMP option it cam be moved to
GENERIC SMP config. SMP itself does not work on RPi3 yet due to lack of
PSCI monitor which is work in progress at the moment

sys/arm64/conf/GENERIC

@@ -90,6 +90,7 @@ options 	MALLOC_DEBUG_MAXZONES=8	# Separate malloc(9) zones
 options 	SOC_ALLWINNER_A64
 options 	SOC_CAVM_THUNDERX
 options 	SOC_HISI_HI6220
+options 	SOC_BRCM_BCM2837
 
 # Annapurna Alpine drivers
 device		al_ccu			# Alpine Cache Coherency Unit

sys/arm64/conf/GENERIC-UP

@@ -21,5 +21,3 @@
 include		GENERIC
 ident		GENERIC-UP
 nooptions 	SMP
-
-options 	SOC_BRCM_BCM2837