meta-arm-bsp: add layer dependencies

meta-arm-bsp has dependencies on other layers, which are not being
represented in the layers.conf file.  This causes build breaks when
using tools like `bitbake-layers layerindex-fetch`, as it will not fetch
the depencies properly.  Currently, the dependencies on other layers are:
* core => u-boot, linux, etc
* openembedded-layer (meta-oe) => haveged
* meta-arm => trusted-firmware-a

Change-Id: Icafc2f1c55a6138172706bb4b99e9dc59904cad4
Signed-off-by: Jon Mason <jon.mason@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>