These file changes provide small fixes for various targets
on MPS2 platform in order to work properly and pass all
Greentea test cases. The __initial_sp has been explicitly set
in the targets' startup files, and also INITIAL_SP has been
given a different value. The values have been extracted from
the specific targets' Application Note documentation.
Affected targets are: ARM_MPS2_M0, ARM_MPS2_M0P, ARM_MPS2_M3,
ARM_MPS2_M4 and ARM_MPS2_M7.

Change-Id: I3d5d0e1ae386cdcc3ba5eb63be929267a257b139
Signed-off-by: Bence Kaposzta <bence.kaposzta@arm.com>