rockchip: rk3399: do not generate u-boot.rom anymore

rockchip: rk3399: do not generate u-boot.rom anymore

This was only used on RK3399 Gru Chromebooks and their maintainer
(Simon) agreed[1] to its removal on the basis that the generic
u-boot-rockchip-spi.bin is now enough, so let's do that.

At the same time, remove HAS_ROM symbol from the Gru Chromebooks config
since they were used only for that.

Make sure u-boot-rockchip-spi.bin has the same size of u-boot.rom for
Chromebooks as that seems to be important.

[1] https://lore.kernel.org/u-boot/CAFLszTh-SewFod8dEOF3+e-wCE1qFF0CyxxR8CbQwy3BRW3k6w@mail.gmail.com/

Reviewed-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Simon Glass <sjg@chromium.org>  # chromebook-kevin
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>