Included cot_descriptors.dtsi in platform device tree
(fvp_tb_fw_config.dts).

Also, updated the maximum size of tb_fw_config to 0x1800
in order to accomodate the device tree for CoT descriptors.

Follow up patch will parse the device tree for these CoT descriptors
and fill the CoT descriptor structures at runtime instead of using
static CoT descriptor structures in the code base.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I90122bc713f6842b82fb019b04caf42629b4f45a