Revert "rockchip: Allow booting from SPI"

This reverts commit 3523c078.

Booting from SPI was already allowed before this commit was first
introduced. A few lines further down the exact same code already existed
and still does.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>