tables.h: make DECLARE_LINKTABLE() .data by default

Upon review with hpa its better to have the shorter version for
what we expect linker tables to be used for mostly, and that is data.
DECLARE_LINKTABLE_TEXT() is then required for .text.

Expand documentation on that we have then only two declarations
also:

  o DECLARE_LINKTABLE()        for data
  o DECLARE_LINKTABLE_RO()     for read-only data

The definitions are used to associate these with actual sections
on the code that implements support for the table.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
4 files changed