serial: msm_geni_serial: Reduce nos HSUART port supported

Currently UART serial driver allocates totally 15 HSUART  ports which
is very high and results in unwanted memory and allocation process.
As of now overall HSUART usecases are limited to Max 3 at a time,
hence keep the ports minimally meeting the usecase requirements.

Change-Id: I4979084dc6ada7a904b3f14c1ee175b78e98b780
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>