ARM: omapl138_lcdk: Allow early init to start instruction cache

Currently the omapl138_lcdk has SKIP_LOWLEVEL_INIT set.
The README states there is a variation of this for the ARM926EJ-S
which allows the board to just skip the call to lowlevel_init()
and do the normal CP15 init which enables the instruction cache.

On the da850evm, this was shown to improve startup time.

This patch switches SKIP_LOWLEVEL_INIT to SKIP_LOWLEVEL_INIT_ONLY
thus, enabling the cache.

Signed-off-by: Adam Ford <aford173@gmail.com>
Tested-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>