ARM: dts: aspeed: bletchley: Rename flash1 label

In OpenBMC phosphor-software-manager uses "alt-bmc" for the secondary flash
label.

Rename flash1 label to "alt-bmc" to support the dual image feature in OpenBMC.

Signed-off-by: Potin Lai <potin.lai.pt@gmail.com>
Reviewed-by: Patrick Williams <patrick@stwcx.xyz>
Link: https://lore.kernel.org/r/20221226054535.2836110-2-potin.lai.pt@gmail.com


Signed-off-by: Joel Stanley <joel@jms.id.au>