configs: rpi_arm64: sync env size with rpi_{3,4}_defconfig

Use the same environment size as the configs dedicated for rpi3 and rpi4.
This allows to switch between the builds and not to loose the settings
stored on the SD card.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>