morello: Add build option to use sensor library

Morello SoC platform specific build option,
SCP_MORELLO_SENSOR_LIB_PATH has been added which
imports the morello sensor library.

Also, added a readme.md file which describes this option.

Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>
Change-Id: Ic4b9754fbd10974250fe19d4c4f381cb1e74899f