feat(plat/rcar3): modify type for Internal function argument

Modify the type of the variable that stores the value for MPIDR
in the internal function from uint64_t to u_register_t.

Signed-off-by: Koichi Yamaguchi <koichi.yamaguchi.zb@hitachi.com>
Signed-off-by: Toshiyuki Ogasahara <toshiyuki.ogasahara.bo@hitachi.com>
Signed-off-by: Yoshifumi Hosoya <yoshifumi.hosoya.wj@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Change-Id: Ib5bda93d5432e0412132bddf41ead8ee3fcf9e46