arm: dts: fix ast2500-evb inclusion for the correct soc family

arm: dts: fix ast2500-evb inclusion for the correct soc family

Include ast2500-evb.dtb for CONFIG_ASPEED_AST2500 instead of
for all aspeed targets.

ast2400 is based on ARM926EJ-S processor (ARMv5-architecture).
ast2500 is based on ARM1176JZS processor (ARMv6-architecture).
ast2600 is based on Cortex A7 processor (ARMv7-A architecture).
Each of the above SOC is using a different ARM CPU(s) with different ARM
architecture revision. It is not possible to support all 3 of these
families in a single binary. So there is no need to build ast2500-evb.dtb
for other SOC families.

Signed-off-by: Thirupathaiah Annapureddy <thiruan@linux.microsoft.com>