SATA: do not auto-initialize during boot

Rather than have the board code initialize SATA automatically during boot,
make the user manually run "sata init".  This brings the SATA subsystem in
line with common U-Boot policy.

Rather than having a dedicated weak function "is_sata_supported", people
can override sata_initialize() to do their weird board stuff.  Then they
can call the actual __sata_initialize().

Signed-off-by: Mike Frysinger <vapier@gentoo.org>