ARM: dts: silk: Name serial port pfc subnode after device name

Name the Pin Function Controller subnode for SCIF2 after its device
name, instead of using some arbitrary name that looks like a serial port
alias, but differs from the actual alias.

This avoids conflicts when enabling support for more serial ports later,
either here or in a DT overlay.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>