xlat_tables_v2: add base table section name parameter for spm_mm

Core spm_mm code expects the translation tables are located in the
inner & outer WBWA & shareable memory.
REGISTER_XLAT_CONTEXT2 macro is used to specify the translation
table section in spm_mm.

In the commit 363830df (xlat_tables_v2: merge
REGISTER_XLAT_CONTEXT_{FULL_SPEC,RO_BASE_TABLE}), REGISTER_XLAT_CONTEXT2
macro explicitly specifies the base xlat table goes into .bss by default.
This change affects the existing SynQuacer spm_mm implementation.
plat/socionext/synquacer/include/plat.ld.S linker script intends to
locate ".bss.sp_base_xlat_table" into "sp_xlat_table" section,
but this implementation is no longer available.

This patch adds the base table section name parameter for
REGISTER_XLAT_CONTEXT2 so that platform can specify the
inner & outer WBWA & shareable memory for spm_mm base xlat table.
If PLAT_SP_IMAGE_BASE_XLAT_SECTION_NAME is not defined, base xlat table
goes into .bss by default, the result is same as before.

Change-Id: Ie0e1a235e5bd4288dc376f582d6c44c5df6d31b2
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>