ARM: dts: qcom: Add vendor directory to the Makefile

Add vendor directory to the devicetree makefile. This directory
will be included in the compilation only if it's present.

Change-Id: Ib644d00fd61469afa2311d214866607b78220f71
Signed-off-by: Venkata Narendra Kumar Gutta <vnkgutta@codeaurora.org>