Make the header guards more generic and contextually remove the
GXBB_BL31_PLAT_PARAM_VAL value that is unused on the GXL platform.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Change-Id: I842fa2e084e71280ae17b39c67877e844821a171

The code is the common directory is now generic, no need to have the SoC
prefix hardcoded in the function names.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Change-Id: Ied3a5e506b9abd4c2d6f893bafef50019bff24f1

Meson is the internal code name for the SoC family. The correct name for
the platform should be Amlogic. Change the name of the platform
directory.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Change-Id: Icc140e1ea137f12117acbf64c7dcb1a8b66b345d