omap-common: Common boot code OMAP3 support and cleanup

This introduces OMAP3 support for the common omap boot code, as well as a
major cleanup of the common omap boot code.

First, the omap_boot_parameters structure becomes platform-specific, since its
definition differs a bit across omap platforms. The offsets are removed as well
since it is U-Boot's coding style to use structures for mapping such kind of
data (in the sense that it is similar to registers). It is correct to assume
that romcode structure encoding is the same as U-Boot, given the description
of these structures in the TRMs.

The original address provided by the bootrom is passed to the U-Boot binary
instead of a duplicate of the structure stored in global data. This allows to
have only the relevant (boot device and mode) information stored in global data.
It is also expected that the address where the bootrom stores that information
is not overridden by the U-Boot SPL or U-Boot.

The save_omap_boot_params is expected to handle all special cases where the data
provided by the bootrom cannot be used as-is, so that spl_boot_device and
spl_boot_mode only return the data from global data.

All of this is only relevant when the U-Boot SPL is used. In cases it is not,
save_boot_params should fallback to its weak (or board-specific) definition.
save_omap_boot_params should not be called in that context either.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>