bl2_el3: clean up linker script

This linker script is so unreadable due to sprinkled #ifdef.

Direct read-only data to 'ROM' and read-write data to 'RAM'.

Both go to the same memory device when BL2_IN_XIP_MEM is disabled.

Change-Id: Ieeac3f1a4e05e9e8599de2ec84260819c70f361e
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>