msm_serial_hs : Add support to select device id with BLSP Based UART

UART device is enumerated as /dev/ttyHS<id>. Currently cell-index
is used as id to enumerated the device. It is not required to
provide any device specific configuration with device tree usage.
Hence use below approaches to select device ID with BLSP based UART:

1. Use auto increment approach for selecting id by default. HSUART
driver would increment id based on HSUART device binding with driver.

2. Add support to provide user defined id for particular HSUART device.
Alias property can be used as user defined id for used HSUART device.
Hence add support for the same. Also check if alias based ID is already
under-used, then fail the probe of driver for the same.

Here it is responsibility of user to use either of above approach.
If hybrid approach is being used,then user shouldn't use those
aliases ID which are already used by previously enumerated HSUART
devices.

Add required documentaion for alias usage for BLSP based UART.

CRs-Fixed: 410636
Change-Id: I9d18b047a7d5fa87320f3cb1edfd5d5a287f5061
Signed-off-by: Saket Saurabh <ssaurabh@codeaurora.org>
1 file changed