ARC: Don't source drivers/pci/pcie/Kconfig ourselves

Commit 5f8fc432 ("PCI: Include pci/pcie/Kconfig directly from
pci/Kconfig") in linux-next changed drivers/pci/Kconfig to include
drivers/pci/pcie/Kconfig itself, so that architectures do not need
to source both files themselves. ARC just recently gained PCI support
through commit 6b3fb77998dd ("ARC: Add PCI support"), but this change
was based on the old behaviour of the Kconfig files. This makes
Kconfig now spit out the following warnings:

drivers/pci/pcie/Kconfig:61:warning: choice value used outside its choice group
drivers/pci/pcie/Kconfig:67:warning: choice value used outside its choice group
drivers/pci/pcie/Kconfig:74:warning: choice value used outside its choice group

This change updates the Kconfig file for ARC, dropping the now
unnecessary 'source' statement, which makes the warning disappear.

Signed-off-by: Andreas Ziegler <andreas.ziegler@fau.de>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>