msm_serial_hs: Fix the logic for numbering the serial device

This patch fixes the way the UART nodes are enumerated when
aliases are used to identify the nodes. The atomic variable that
used to hold the next available node number was incorrectly being
incremented and if that number were to hold a number that was also
used in an alias, the probe would fail. This patch corrects it.
It also removes the atomic variable and introduces two apis to set
and get the next device number and this logic is protected by a mutex.

Change-Id: Iaee655603b527f3283e1f12d90b1220e9315b529
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>