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.