arm: Allow skipping of low-level init with I-cache on

arm: Allow skipping of low-level init with I-cache on

At present CONFIG_SKIP_LOWLEVEL_INIT prevents U-Boot from calling
lowlevel_init(). This means that the instruction cache is not enabled and
the board runs very slowly.

What is really needed in many cases is to skip the call to lowlevel_init()
but still perform CP15 init. Add an option to handle this.

Reviewed-by: Heiko Schocher <[email protected]>
Tested-on: smartweb, corvus, taurus, axm
Tested-by: Heiko Schocher <[email protected]>
Reviewed-by: Joe Hershberger <[email protected]>
Signed-off-by: Simon Glass <[email protected]>
Reviewed-by: Andreas Bießmann <[email protected]>