arm: rmobile: r8a7794: Skip initialize L2 cache

rmobile/lowlevel_init_ca15.S are common in r8a7790, r8a7791 and r8a7794 of
rmobile SoCs.  The initialize L2 cache in lowlevel_init_ca15.S only needed
for Cortex-A15. The r8a7794 is Cortex-A7, not Cortex-A15.
This adds Skip to initialize L2 cache when r8a7794.

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