I don't think you can. You either boot off a flexbus device or using the Serial Boot Flash (SBF) over SPI.
The two signals that control booting are BOOTMOD[1:0] (section 10.2.1 in the RM). Your options are:
- Boot from Flexbus with defaults
- Boot from Flexbus and override defaults via data bus
- Boot from Flexbus and override defaults via the serial boot facility
- Reserved (just for those wondering about the last bit combination)
You can't go directly to SDRAM. Also you'd need to setup the SDRAM controller before you could execute code from it.
--Kurt