am33xx: board: Refactor USB initialization into separate function

am33xx: board: Refactor USB initialization into separate function

The declaration of otg*_plat and otg*_board_data is guarded by
CONFIG_USB_MUSB_*, but their use in arch_misc_init is not. The
ifdef flow goes something like:

if (CONFIG_USB_MUSB_* && other_conditions)
	declare usb_data
if (other_conditions)
	use usb_data

Thus when CONFIG_USB_MUSB_* is not declared, we try to use the
data structures, but these structures aren't defined.

To fix this, move the USB initialization code into the same #ifdef
which guards the declaration of the data structures. Since the DM_USB
vs legacy cases are completely different, use two versions of
arch_misc_init(), for readability.

Signed-off-by: Alexandru Gagniuc <alex.g@adaptrum.com>
Reviewed-by: Tom Rini <trini@konsulko.com>