If the code is already executing in DRAM, the PS7 init must not be
executed, as it initializes the DRAM controller.

As the OCM can be configured to an address which aliases with the DRAM
address space we can't reliably infer if we are running from OCM or
DRAM from the execution address. So instead of using the address, look
at the OCM mapping, as the BootROM leaves a quite unique mapping behind
with 192KB OCM mapped at the low address and 64KB mapped to the high
address.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>