ARM: da850-evm: Fix Lowlevel init

ARM: da850-evm: Fix Lowlevel init

A previous patch for enabling the NAND config set a flag called
CONFIG_SKIP_LOWLEVEL_INIT when it should have been called
CONFIG_SKIP_LOWLEVEL_INIT_ONLY.  The affect this had was creating
a delay on startup for the NOR version which is XIP and doesn't have
SPL, so the lowlevel initialization functions need to operate.
This delay was not really noticeable at first, but the delays have been
getting longer, finally reached the point of nearly seven seconds
before the board would appear to start.

This patch sets the CONFIG_SKIP_LOWLEVEL_INIT_ONLY instead which means
"The normal CP15 init (such as enabling the instruction cache) is still
performed" per the README.  It doesn't appear to have any adverse
behavior on the SPI Flash or the NAND flash boards which use SPL.

Fixes: 93f3362762f0 ("ARM: configs: Add da850evm_nand to boot from NAND")

Signed-off-by: Adam Ford <aford173@gmail.com>