Move BL1 and BL2 private defines to bl_common.h

The definitions in bl1/bl1_private.h and bl2/bl2_private.h are useful for
platforms that may need to access them.

Change-Id: Ifd1880f855ddafcb3bfcaf1ed4a4e0f121eda174
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>