Factor xlat_table sections in linker scripts out into a header file

TF-A has so many linker scripts, at least one linker script for each BL
image, and some platforms have their own ones. They duplicate quite
similar code (and comments).

When we add some changes to linker scripts, we end up with touching
so many files. This is not nice in the maintainability perspective.

When you look at Linux kernel, the common code is macrofied in
include/asm-generic/vmlinux.lds.h, which is included from each arch
linker script, arch/*/kernel/vmlinux.lds.S

TF-A can follow this approach. Let's factor out the common code into
include/common/bl_common.ld.h

As a start point, this commit factors out the xlat_table section.

Change-Id: Ifa369e9b48e8e12702535d721cc2a16d12397895
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>