drivers: pcie_ep: iproc: Remove sys_read8 from unmap API

Originally, sys_read8 to the mapped address was added part of
the unmap API to protect OMAP mapping, i.e. in case of PCIe writes,
OMAP should not be overwritten before writes are completed.

Now that we have added dummy PCIe read in the common APIs, namely
pcie_ep_xfer_data_memcpy and pcie_ep_xfer_data_dma, for the purpose
of flushing PCIe writes; the purpose of protecting OMAP *also*
gets satisfied, randering PCIe read in the unmap useless,
so remove the same.

Signed-off-by: Abhishek Shah <abhishek.shah@broadcom.com>