uio: Add driver for inter-VM shared memory device

This adds a UIO driver the ivshmem device, found in QEMU and the
Jailhouse hypervisor. It exposes the MMIO register region and all shared
memory section to userspace. Interrupts are configured in one-shot mode
so that userspace needs to re-enable them after each event via the
Interrupt Control register. The driver registers all possible MSI-X
vectors, coalescing them into the single notifier UIO provides.

Note: Specification work for the interface is ongoing, so details may
still change.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Nikhil Devshatwar <nikhil.nd@ti.com>