Blackfin: add serial TX IRQ in individual platform resource

The serial TX IRQ is not simply (RX IRQ + 1) on some Blackfin chips,
so move the values to the platform resources.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Bob Liu <lliubbo@gmail.com>