hw/misc/aspeed_xdma: Add AST2600 support

hw/misc/aspeed_xdma: Add AST2600 support

When we introduced support for the AST2600 SoC, the XDMA controller
was forgotten. It went unnoticed because it's not used under emulation.
But the register layout being different, the reset procedure is bogus
and this breaks kexec.

Add a AspeedXDMAClass to take into account the register differences.

Cc: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Eddie James <eajames@linux.ibm.com>
Message-Id: <20210407171637.777743-14-clg@kaod.org>
Signed-off-by: Cédric Le Goater <clg@kaod.org>