rockchip: defconfig: puma-rk3399: bypass ADC-based boot_mode check

The boot (and fallback/emergency boot) concept for the RK3399-Q7
differs from Rockchip's reference platforms.

On the RK3399-Q7, some of this functionality is present in the
bootloader itself (and configurable); some is backed in hardware by
the Qseven BIOS_DISABLE signal to invoke the final stages of fallbacks
(i.e. either an external boot bypassing on-module memories or falling
back to the BROM for USB recovery).

In summary: the ADC-based boot_mode check does not apply for the
RK3399-Q7 and we therefore disable it (in this commit) by setting
CONFIG_BOOT_MODE_REG to 0.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>