linux-firmware: update firmware for mhdp8546

This patch updates mhdp8546 firmware from v1.2.12 to v1.2.15.

Added support for performing arbitrary I2C-over-AUX transactions.
Corrected handling of erroneous requests for reading/writing 0 bytes over DPCD.
Add mailbox message to force uCPU fatal error.

Signed-off-by: Damian Kos <dkos@cadence.com>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>

WHENCE

@@ -4383,7 +4383,7 @@ Licence:
 Driver: cdns-mhdp - Cadence MHDP DP bridge
 
 File: cadence/mhdp8546.bin
-Version: 1.2.12
+Version: 1.2.15
 
 Licence: Redistributable. See LICENCE.cadence for details