am335x: Enable SPL_OF_CONTROL

It has long been known that we need to enable SPL_OF_CONTROL on this
platform, as some included drivers do not function correctly without it,
but were also stuck due to size constraints.  Resolve this problem by:

- Updating the generic board.c file to use CONFIG_IS_ENABLED(USB_ETHER)
  to distinguish between SPL and full U-Boot.
- Disable SPL_ETH support as the CPSW driver (and related PHY/etc) are
  were we need SPL_OF_CONTROL support, but cannot due to size constraints.
- Update the documentation on how to enable this mode and recommend what
  to remove in order to fit within size constraints.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
3 files changed