opensbi: bump to 1.6

opensbi: bump to 1.6

Upgrade the OpenSBI firmware used by RISC-V CPUs to 1.6.

Runtime-tested:
 - d1 (LicheeRV Dock)
 - sifiveu (SiFive Unleashed)

Updates since last release:

1.6:
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

1.5.1:
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()

1.5:
SBI debug triggers (DBTR) extension (Experimental)
Support to specify coldboot harts in DT
Relocatable FW_JUMP_ADDR and FW_JUMP_FDT_ADDR
Smcsrind and Smcdeleg extensions support
SBIUnit testing framework
Initial domain context management support
Platform specific load/store emulation callbacks
New trap context
Improved sbi_trap_error() to dump state in a nested trap
SBI supervisor software events (SSE) extension (Experimental)
Simplified wait_for_coldboot() implementation
Early wakeup of non-coldboot HART in the coldboot path
Sophgo CV18XX/SG200X series support
APLIC delegation DT property fix
Svade and Svadu extensions support
SBI firmware features (FWFT) extension (Experimental)

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>