Minor changes to documentation and comments ...

Fix some typos and clarify some sentences.

Change-Id: Id276d1ced9a991b4eddc5c47ad9a825e6b29ef74
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

plat/arm: sds: Move to drivers/ folder ...

Change-Id: Ia601d5ad65ab199e747fb60af4979b7db477d249
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

plat/arm: scp: Move to drivers/ folder ...

Change-Id: Ida5dae39478654405d0ee31a6cbddb4579e76a7f
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

plat/arm: scpi: Move to drivers/ folder ...

Change-Id: Icc59cdaf2b56f6936e9847f1894594c671db2e94
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

plat/arm: mhu: Move to drivers/ folder ...

Change-Id: I656753a1825ea7340a3708b950fa6b57455e9056
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>

plat/arm: scmi: Move to drivers/ folder ...

Change-Id: I8989d2aa0258bf3b50a856c5b81532d578600124
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>