arm64: renesas: r8a779g3: Use $loadaddr in bootcmd on Retronix R-Car V4H Sparrow Hawk board

Avoid use of hard-coded address in boot command, instead use $loadaddr
which is the default load address. This improves consistency of the
environment on this board.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>