arm/ast2600: Fix SMP booting with -kernel

The ast2600 machines do not have PSCI firmware, so this property should
have never been set. Removing this node fixes SMP booting Linux kernels
that have PSCI enabled, as Linux fails to find PSCI in the device tree
and falls back to the soc-specific method for enabling secondary CPUs.

The comment is out of date as Qemu has supported -kernel booting since
9bb6d140 ("aspeed: Add boot stub for smp booting"), in v5.1.

Fixes: f25c0ae1 ("aspeed/soc: Add AST2600 support")
Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20210303010505.635621-1-joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>