[arch][arm] Add missing isb to arch_enable_cache
[arch][arm] Add missing isb to arch_enable_cache

Without this load/store-exclusive can fail on systems where
these operations are only supported in cache-able memory.

Change-Id: I7301e430cc66d92a20c876bdf56da2aa24babd13

grub4android/lk,minglun-tsai/lk,nvll/lk,ErikCorryGoogle/lk,skabet/lk,skabet/lk,nvll/lk,ErikCorryGoogle/lk,skabet/lk,sndnvaps/lk,nvll/lk,minglun-tsai/lk,GCrean/lk,GCrean/lk,minglun-tsai/lk,srodrig1/lk,nvll/lk,srodrig1/lk,sndnvaps/lk,hollanderic/lkstuff,travisg/lk,travisg/lk,grub4android/lk,grub4android/lk,travisg/lk,littlekernel/lk,sndnvaps/lk,hollanderic/lkstuff,littlekernel/lk,ErikCorryGoogle/lk,GCrean/lk,littlekernel/lk,skabet/lk,grub4android/lk,srodrig1/lk,sndnvaps/lk,minglun-tsai/lk,grub4android/lk,travisg/lk,GCrean/lk,sndnvaps/lk,srodrig1/lk,GCrean/lk,skabet/lk,hollanderic/lkstuff,nvll/lk,littlekernel/lk,srodrig1/lk,ErikCorryGoogle/lk,hollanderic/lkstuff