arm: rmobile: r8a7791: Fix initialize L2 cache

rmobile/lowlevel_init_ca15.S are common in r8a7790 and r8a7791 of
rmobile SoC. But L2 cache of r8a7791 does not use L2CTLR[5].
This adds fix to set L2CTLR [5] only when the r8a7790.

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