xlat_tables_v2: merge REGISTER_XLAT_CONTEXT_{FULL_SPEC,RO_BASE_TABLE}

xlat_tables_v2_helpers.h defines two quite similar macros,
REGISTER_XLAT_CONTEXT_FULL_SPEC and REGISTER_XLAT_CONTEXT_RO_BASE_TABLE.

Only the difference is the section of _ctx_name##_base_xlat_table.

Parameterize it and unify these two macros.

The base xlat table goes into the .bss section by default.
If PLAT_RO_XLAT_TABLES is defined, it goes into the .rodata section.

Change-Id: I8b02f4da98f0c272e348a200cebd89f479099c55
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>