refactor(corstone700): namespace MHU driver filenames

refactor(corstone700): namespace MHU driver filenames

There are plans to contribute a generic MHU driver to the TF-A code
base in the short term.

In preparation for this, rename the Corstone-700 MHU driver source
files and prefix them with the name of the platform to avoid any
ambiguity or name clashes with the upcoming generic MHU driver. Also
rename the header guard accordingly.

This renaming is inline with other platform-specific MHU drivers, such
as the ones used on Broadcom [1], Socionext [2] or Amlogic [3] platforms.

[1] plat/brcm/common/brcm_mhu.h
[2] plat/socionext/synquacer/drivers/mhu/sq_mhu.h
[3] plat/amlogic/common/aml_mhu.c

Change-Id: I8a5e5b16e7c19bf931a90422dfca8f6a2a0663b4
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>