defconfigs: am43xx_hs_evm: Add USB Host boot mode support

Enable SPL_USB_HOST_SUPPORT in the default defconfig to allow
booting from USB peripherals. Unlike the non-HS boards, we
already load SPL to a 0x4030_0000+ address, so no other changes
are needed.

Signed-off-by: Andrew F. Davis <afd@ti.com>