MT8173 platform code is incompatible with RESET_TO_BL31, add #error
directive to prevent the case.

We also move mt8173_def.h and plat_private.h to include directory, and
remove some unnecessary code.

Change-Id: I47b8d0a506820a4ea1fbe8c8fb0ec6c68d88feb5
Signed-off-by: Jimmy Huang <jimmy.huang@mediatek.com>