Add cpu_l2cache_drain_writebuf(), use it to implement generic_bs_barrier().

On modern ARM SoCs the L2 cache controller sits between the CPU and the
AXI bus, and most on-chip memory-mapped devices are on the AXI bus.  We
map the device registers using the 'Device' memory attribute, which means
the memory is not cached, but writes to it are buffered.  Ensuring that a
write has made it all the way to a device may require that the L2
controller take some action.

There is currently only one implementation of the new function, for the
PL310 cache controller.  It invokes a function that the controller
manual calls "cache sync" but it actually has nothing to do with cache at
all, it triggers a drain of all pending store buffer writes and it blocks
until they complete.

The sheeva and xscale L2 controllers (which predate the concept of Device
memory) don't seem to have a corresponding function.  It appears that the
standard armv5 drain_writebuf function includes draining all the way
through the L2 controller.