arm64: Juno/FVP: adjust kernel load address

The default kernel load offset for an arm64 kernel is 0x80000, so
U-Boot takes cares of moving the loaded kernel to a matching memory
location just before booting it.
Since we run with caches off, this takes a while for any decently
sized kernel - with no output explaining the reason for the delay
(unless one uses a DEBUG build).
By adjusting the default load offset for Juno and VFP to be 512K
aligned in the first place we can skip this copying and boot much
faster.
Tested on Juno.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>