configs: rk3588-turing-rk1: disable SPI flash by default

While the Turing RK1 board has a pad on the PCB for SPI flash, it is
not populated at the factory: supporting SPI flash boot is a user
modification, not an out-of-the-box feature. The defconfig for this
board should therefore not be enabling the SPI flash image nor SPI
support in the SPL, as it causes confusion among downstream users as to
whether the SPI image needs to be distributed.

Fixes: 153ac950 ("board: rockchip: Add the Turing RK1 SoM")
Suggested-by: Florian Klink <flokli@flokli.de>
Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Acked-by: Joshua Riek <jjriek@verizon.net>
Reviewed-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>