target/hppa: Update SeaBIOS-hppa to version 15

SeaBIOS-hppa version 15:
- Fix OpenBSD 7.4 boot (PDC_MEM_MAP call returned wrong values)

SeaBIOS-hppa version 14 comes with those fixes:
- Fix 32-bit HP-UX crash (fix in PDC_FIND_MODULE call)
- Fix NetBSD boot (power button fix and add option to disable it)
- Fix FPU detection on NetBSD
- Add MEMORY_HPA module on B160L
- Fix detection of mptsas and esp scsi controllers
- Fix terminate DMA transfer in esp driver (Mark Cave-Ayland)
- Allow booting from esp controller

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