arm: rmobile: bugfix: wrong register saving in lowlevel_init

lowlevel_init() of rmobile badly assumed that ip register holds return address.
The commit "63ee53a7 armv7 cpu_init_crit: Simplify code" breaks this assumption.
This patch removes this bad assumption and simplify code.

Signed-off-by: Tetsuyuki Kobayashi <koba@kmckk.co.jp>