eal: introduce driver init macros

Introduce a RTE_INIT macro used to mark an init function as a constructor.
Current eal macros have been converted to use this (no functional impact).
DRIVER_REGISTER_PCI is added as a helper for pci drivers.

Suggested-by: Jan Viktorin <viktorin@rehivetech.com>
Signed-off-by: David Marchand <david.marchand@6wind.com>
[Shreyansh: Update PCI Registration macro name]
Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>