staging: vchiq: Move vchiq char driver to its own file

Split the initialization code of vchiq char driver and device files from
that of vchiq platform. The char driver code now resides in vchiq_dev.c
and the platform code resides in the original vchiq_arm.c file.

This commit focuses on separating the code into different files while
maintaining the same functionality. It does not completely decouple them
as the cdev init code is still called from the platform's vchiq_probe()
function.

Reviewed-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Ojaswin Mujoo <ojaswin98@gmail.com>
Link: https://lore.kernel.org/r/ed53690e217f631b746aad0585fccaa6fb1453c1.1626882325.git.ojaswin98@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>