Convert CONFIG_USB_MUSB_OMAP2PLUS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_USB_MUSB_OMAP2PLUS
   CONFIG_USB_MUSB_AM35X
   CONFIG_USB_MUSB_DSPS
   CONFIG_USB_MUSB_PIO_ONLY

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index e6c17c7..bb7cc3f 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -40,5 +40,6 @@
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y
 CONFIG_USB_MUSB_HOST=y
+CONFIG_USB_MUSB_AM35X=y
 # CONFIG_FAT_WRITE is not set
 CONFIG_BCH=y