opensbi: bump to 1.5.1

opensbi: bump to 1.5.1

This intermediate release has following fixes:
- Save/restore menvcfg only when it exists
- Adjust Sscofpmf mhpmevent mask for upper 8 bits
- Fix potential NULL pointer dereferences in SBI DBTR
- Fix incorrect size passed to sbi_zalloc() in SBI FWFT
- Check result of pmp_get() in is_pmp_entry_mapped()

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>