PCI: iproc: Hide CONFIG_PCIE_IPROC

PCIE_IPROC_BCMA does not require CONFIG_OF in Kconfig, but
CONFIG_PCIE_IPROC does, so we can get a warning when building for an ARM
platform without DT support:

  warning: (PCIE_IPROC_PLATFORM && PCIE_IPROC_BCMA) selects PCIE_IPROC which has unmet direct dependencies (PCI && OF && (ARM || ARM64))

It turns out that CONFIG_PCIE_IPROC never needs to be enabled by a user
anyway, we can simply rely on it being selected implictly through either
PCIE_IPROC_PLATFORM or PCIE_IPROC_BCMA.

Fixes: 4785ffbd ("PCI: iproc: Add BCMA PCIe driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>