kernel: make tty count for 8250 configurable

kernel: make tty count for 8250 configurable

The number of serial interfaces for the chip 8250 can be configured at
build time. This is currently not configurable and is always set to 16
interfaces. This is too little for some small embedded devices and too much
for others (x86_64). Therefore, this commit creates the possibility to set
this at compile time.

Signed-off-by: Florian Eckert <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19217
Signed-off-by: Hauke Mehrtens <[email protected]>