This is defined based on two other CONFIGs for all boards except sandbox
and durian.

For sandbox the value does not matter. For durian the value seems
excessive.

Drop the option completely, to simplify configuration and reduce the
number of things we need to convert to Kconfig.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>