opensbi: bump to 1.6

opensbi: bump to 1.6

This release has:
- Support for parsing riscv,isa-extensions DT property
- Setup serial console very early in cold boot path
- Support for multiple heaps and aligned memory allocation
- Support for shadow stacks (Zicfiss) ISA extension
- Support for landing pads (Zicfilp) ISA extension
- Support for per-domain data
- Support for double-trap (Smdbltrp/Ssdbltrp) ISA extensions
- DT-based configurable heap size
- Common fdt_driver and helpers for driver initialization
- Support for SBI PMU raw event v2 (Experimental)
- Simple FDT based mailbox driver framework
- RPMI shared memory transport driver (Experimental)
- RPMI system reset driver (Experimental)
- Simple FDT based system suspend driver framework
- RPMI system suspend driver (Experimental)
- Simple FDT based HSM driver framework
- RPMI HSM driver (Experimental)
- Simple FDT based CPPC driver framework
- RPMI CPPC driver (Experimental)
- SBI Message Proxy (MPXY) extension (Experimental)
- Simple FDT based MPXY driver framework
- Common RPMI client driver for MPXY (Experimental)
- Support for vector misaligned load/store

Overall, this release adds more ISA extensions, drivers, and other improvements.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>