Switch the tx_lock to a spinlock we allow clients to use rpmsg_trysend()
from atomic context.

In order to allow clients to sleep while waiting for space in the FIFO
we release the lock temporarily around the delay; which should be
replaced by sending a READ_NOTIF and waiting for the remote to signal
us that space has been made available.
Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>