usb: musb: implement (un)map_urb_for_dma hooks

MUSB controller cannot work in DMA mode with misaligned buffers,
switching in PIO mode.

HCD core has hooks that allow to override the default DMA
mapping and unmapping routines for host controllers that have
special DMA requirements, such as alignment constraints.

It is observed that work in PIO mode is slow and it's better
to align buffers properly before passing them to MUSB

This increased throughput 80->120 MBits/s over musb@omap4 with
USB Gigabit Ethernet adapter attached.

Some ideas are taken from ehci-tegra.c

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>