fix(plat/rcar3): change stack size of BL31

Increase the stack size to avoid stack overflow
when the LOG_LEVEL compile option is set high.

Signed-off-by: Hideyuki Nitta <hideyuki.nitta.jf@hitachi.com>
Signed-off-by: Toshiyuki Ogasahara <toshiyuki.ogasahara.bo@hitachi.com>
Signed-off-by: Yoshifumi Hosoya <yoshifumi.hosoya.wj@renesas.com>
Change-Id: I25047322763bff148dba13848a3a40f4c7cf90b7