dt-bindings: altera: document syscon as fallback for sys-mgr

dt-bindings: altera: document syscon as fallback for sys-mgr

For 32-bit Altera SoCFPGA parts, the sys-mgr uses the syscon as a fallback.

This change addresses this warning from dtbs_check:

sysmgr@ffd08000 (altr,sys-mgr): compatible: 'oneOf' conditional failed, one
must be fixed:
	['altr,sys-mgr', 'syscon'] is too long
	'altr,sys-mgr-s10' was expected
	'altr,sys-mgr' was expected
from schema $id: http://devicetree.org/schemas/soc/altera/altr,sys-mgr.yaml

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>