eal: introduce I/O device memory read/write operations

eal: introduce I/O device memory read/write operations

This commit introduces 8-bit, 16-bit, 32bit, 64bit I/O device
memory read/write operations along with the relaxed versions.

The weakly-ordered machine like ARM needs additional I/O barrier for
device memory read/write access over PCI bus.
By introducing the eal abstraction for I/O device memory read/write access,
The drivers can access I/O device memory in architecture agnostic manner.

The relaxed version does not have additional I/O memory barrier, useful in
accessing the device registers of integrated controllers which
implicitly strongly ordered with respect to memory access.

Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>