tegra: usb: move implementation into right directory

This moves the Tegra USB implementation into the drivers/usb/host
directory. Note that this merges the old
/arch/arm/cpu/armv7/tegra20/usb.c file into ehci-tegra.c. No code
changes, just moving stuff around.

v2: While at it also move some defines and the usb.h header file to make
usb driver usable for Tegra30.
NOTE: A lot more work is required to properly init the PHYs and PLL_U on
Tegra30, this is just to make porting easier and it does no harm here.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>