Increase CONFIG_SERIAL_8250_NR_UARTS from 4 to 8, the current legacy value
is not adequate for embedded systems that use SoCs where it's common to
have a large number of serial ports.

No need to change CONFIG_SERIAL_8250_RUNTIME_UARTS, see commit 9d86719f
("serial: 8250: Allow using ports higher than SERIAL_8250_RUNTIME_UARTS").

The need to increase this value was noticed while working with Toradex
Verdin AM62, this board has 4 serial UART instances available to the user
plus an internal one that is connected to a Bluetooth module. Without this
change the fifth UART connected to the BT module is not instantiated and BT
is not working.

Instead of increasing the number to the bare minimum (5) that would be
required to solve this specific issue, we increase this to 8 which seems a
more reasonable number to have in the defconfig and should cover more valid
use cases.

With this change the kernel image size increases by ~3.2kB. bloat-o-meter
summary: add/remove: 1/1 grow/shrink: 7/0 up/down: 3220/-8 (3212)

Cc: Tony Lindgren <tony@atomide.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Francesco Dolcini <francesco@dolcini.it>
Link: https://lore.kernel.org/r/20231201195732.4931-1-francesco@dolcini.itSigned-off-by: Nishanth Menon <nm@ti.com>