What is Platform Bus in Embedded Systems?
I'm not particularly a Linux or SoC developer, but I think this answer is correct:

Yes, the AMBA bus is the platform bus when it is used.

In /drivers/of/platform.c (line 29) when CONFIG_ARM_AMBA is set then the AMBA bus is used in creating the device tree. CONFIG_ARM_AMBA is set in the ARM config file, but it looks like it is only available on some devices.

There's a concise description of what the platform bus is for in the Linux kernel here.