ARM: dts: aspeed: ast2600: Set arch timer always-on

According to ASPEED, FTTMR010 is not intended to be used in the AST2600.
The arch timer should be used, but Linux doesn't enable high-res timers
without being assured that the arch timer is always on, so set that
property in the devicetree.

The FTTMR010 device is described by set to disabled.

This fixes highres timer support for AST2600.

Fixes: 2ca5646b ("ARM: dts: aspeed: Add AST2600 and EVB")
Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>