Convert CONFIG_AM335X_USB0 et al to Kconfig

This converts the following to Kconfig:
   CONFIG_AM335X_USB0
   CONFIG_AM335X_USB0_MODE
   CONFIG_AM335X_USB1
   CONFIG_AM335X_USB1_MODE

We do this by introducing specific options for static configuration of
USB0/USB1 in SPL rather than defining CONFIG_AM335X_USBx_MODE to the
enum value being used.  Furthermore, with how the code is used now we do
not need to have OTG mode exposed as an option here, so remove that.

Signed-off-by: Tom Rini <trini@konsulko.com>