Make housekeeping SPI CSB pin a pullup input on power-up.

[RTimothyEdwards]

The CSB pin is currently set to be a standard management-controlled input on startup (caravel.v line 774, 13'h0403). Instead, the middle one of this array of 3 gpio_defaults_block instances should be 13'h0403, corresponding to a weak pull-up. This ensures that the housekeeping SPI is in a known state on power-up even if nothing is connected to the CSB line. On the development boards, CSB is connected to the FTDI chip which will tri-state this line if not configured for SPI communication.