eal: add internal function to get base address

eal: add internal function to get base address

Added an internal helper to get OS-specific EAL mapping base address

This helper can be used by the drivers to program offload / accelerator
devices, where the base address can be used as a reference address by
the accelerator to access the host memory

An address can also be represented as an offset relative to the base
address using smaller data types

Signed-off-by: Srikanth Yalavarthi <syalavarthi@marvell.com>
Acked-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>