elgin-rv1108: Avoid adc_channel_single_shot error

elgin-rv1108: Avoid adc_channel_single_shot error

Currently the following error message is seen during boot:

U-Boot 2020.01-08751-g55759ae141 (Mar 09 2020 - 14:44:52 -0300)

Model: Elgin RV1108 R1 board
DRAM:  128 MiB
APLL: 600000000 DPLL:1200000000 GPLL:1188000000
ACLK_BUS: 148500000 ACLK_PERI:148500000 HCLK_PERI:148500000 PCLK_PERI:74250000
MMC:   dwmmc@30110000: 0
Loading Environment from MMC... OK
In:    serial@10210000
Out:   serial@10210000
Err:   serial@10210000
Model: Elgin RV1108 R1 board
rockchip_dnl_key_pressed: adc_channel_single_shot fail!
....

Since the elgin-rv1108 does not use ADC to read the download
key status, select CONFIG_ROCKCHIP_BOOT_MODE_REG=0 to avoid
such error.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>