Add documentation about Vendor Methods

This patch adds a quick tutorial on writing vendor specific methods to
a driver.

Implements: blueprint extended-vendor-passthru
Change-Id: Id23e73715e0058ce162a5dde8eeb3ad61d9e7131
Update drivers VendorInterface validate() method

Update drivers VendorInterface validate() methods to remove the
verification of whether a method called exist or not. This verification
is now done by the conductor, validate() will always be called with a
valid method for drivers that are decorating their vendor methods with
the @passthru decorator.

Change-Id: Ia542df8283007ea2a2a049311a6418313a5859b4
Merge "Add documentation about Vendor Methods"
Merge "Update drivers VendorInterface validate() method"