board_r: Introduce CONFIG_PCI_INIT_R Kconfig option

With CONFIG_DM_PCI enabled, PCI buses are not enumerated at boot, as they
are without that config option enabled. However, there are cases such as DM
PCI-based Ethernet devices that need the PCI bus enumerated so that they
can be discovered by their drivers.

Currently, to solve this, some boards enumerate the pci bus using
"pci enum" preboot command, while others do it manually in board files
(in board_init/board_late_init/etc. functions).

In order to possibly make the pci enumeration process uniform across all
boards, introduce CONFIG_PCI_INIT_R Kconfig option.

This change also preserves the current behavior in the !DM_PCI case
(pci_init is run unconditionally at boot).

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>