Blackfin: bf54x: add kconfig for UART2/3 DMA channel assignments

The BF54x lacks dedicated DMA channels for the UART peripherals and need
to be muxed between others.  So add a kconfig option so people can select
which channels the UARTs will use so they can pick between SPORTs and the
less commonly used EPPI/PIXC peripherals.

Signed-off-by: steven miao <realmz6@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>