ARM: renesas: Turn on PIE for Gen3

Turn on PIE, so that the U-Boot binary can be started from any arbitrary
location in DRAM instead of a predefined fixed one. Note that this patch
is not setting SYS_TEXT_BASE=0x0 yet, since that triggers relocation bugs
in env code that are yet to be fixed.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>