etamin: Rework CONFIG_NAND_CS_INIT

Enable this in the board Kconfig file, but then check for it via
CONFIG_IS_ENABLED so that it will only be true in the non-SPL case, as
is done today.  As part of this we move some defines local to where
they are used as it's board specific.

Cc: Samuel Egli <samuel.egli@siemens.com>
Signed-off-by: Tom Rini <trini@konsulko.com>