⚠️ [XIP] reworked execute in place module

[stnolting]

This PR is a rework of the processor's Execute in Place (XIP) module (#244).

• Control register changes
  • removed QSPI mode enable (not implemented yet 😉)
  • rearranged flags
  • removed XIP mode busy flag
  • added flag to explicitly enable the module's SPI chip-select port (allows to send dummy clocks without actually enabling the SPI flash)

Further hardware changes:

• removed hardware-based sending of dummy SPI clocks (is now done by the XIP's software library functions)
• removed "inter-transmission" pause from SPI PHY - faster SPI accesses
• reduces hardware footprint: only ~300 LEs on an Intel Cyclone IV

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⚙️ "Final" pre-defined software functions for using the XIP module:

int neorv32_xip_available(void);
int neorv32_xip_init(uint8_t prsc, uint8_t cpol, uint8_t cpha, uint8_t rd_cmd);
int neorv32_xip_start(uint8_t abytes, uint32_t page_base);
int neorv32_xip_spi_trans(uint8_t nbytes, uint64_t *rtx_data);

💡 A demo program using the XIP module is available in sw/example/demo_xip/main.c.