Platform/RaspberryPi: Expand locking to cover return data

It appears that the locking for many of the mailbox commands is
incorrect. All UEFI firmware calls to the RPi mailbox share a single
mDmaBuffer. That buffer is used to fill out the command passed to the
vc firmware, and record its response. The buffer is protected by
mMailboxLock, yet in many cases the mailbox response is copied from
the buffer after the lock has been released. This doesn't currently
appear to be causing any problems, but should be fixed anyway.

There are a couple other minor tweaks in this patch that are hard to
justify on their own, one is a bit of whitespace cleanup, and the
other is the addition of a debug message to print the returned clock
rate for the requested clock. This latter print would have immediatly
shown that the vc firmware was returning 0 as the emmc clock rate
rather than something reasonable.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>