Boot Source Selection
When pin nRESET transited from low to high, the value of pin PG[1:0] latched to BTSSEL.
00= Boot from USB.
01= Boot from SD/eMMC.
10= Boot from NAND Flash.
11= Boot from SPI Flash.
第三位代表QSPI0CKSEL,1为75Mhz,0为37.5Mhz, 检查CLK是否正常?(供电,相关电阻,电容)