ARM: arm720t: Add missing CONFIG_SKIP_LOWLEVEL_INIT guard for cpu_init_crit

cpu_init_crit() can be skipped, but the code is still enabled requiring a
platform to supply lowlevel_init().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>