This tag does not expose an SPI but a UART given it is not disabled in the CCFG (Customer Configuration) settings.
To read or write the flash you need a 3,3V USB to UART adapter:
D/L
DTR
RTS
Reset
$> ./cc2538-bsl.py CC2630 PG2.3 (7x7mm): 128KB Flash, 20KB SRAM, CCFG.BL_CONFIG at 0x0001FFD8 Primary IEEE Address: 00:12:4B:DE:AD:BE:EF:D9
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