target/hppa: Update SeaBIOS-hppa to version 18

target/hppa: Update SeaBIOS-hppa to version 18

This is SeaBIOS for the hppa architecture v18.
It allows us to emulate up to 256 GB RAM on 64-bit
guests and to boot HP-UX 64-bit one step further.

Fixes:
- Fix PDC_CACHE/PDC_CACHE_RET_SPID return value for space
  register hashing on 64-bit HP-UX
- Fix IRT table entries to use slot number
- Increase PCI alignment for memory bars to 64k

New PDC functions & general enhancements:
- Allow up to 256 GB RAM on 64-bit machines

Signed-off-by: Helge Deller <deller@gmx.de>