ARM: renesas: grpeach: Align env position

Move the U-Boot environment to 0x80000 to match the downstream
vendor fork and allow easy migration from one to the other.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Chris Brandt <chris.brandt@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>