arm: rmobile: r8a7790: Update initialize L2 cache

Initialization of L2CTLR[5] was set only as R8A7790 by commit
237faf09.
However, initialization of cash needs to be performed continuously.
This changes into the processing which continues initialization of
L2CTLR[5] into L2CTLR cash and performs it.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>