Currently, the end address macros are defined per BL, like BL2_END,
BL31_END, BL32_END. They are not handy in the common code shared
between multiple BL stages.

This commit introduces BL_END, which is equivalent to BL{2,31,32}_END,
and will be useful for the BL-common code.

Change-Id: I3c39bf6096d99ce920a5b9fa21c0f65456fbfe8a
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>