ARM: dts: silk: Name spi pfc subnode after device name

Name the Pin Function Controller subnode for QSPI after its device name,
instead of after the spi interface alias.

This avoids conflicts when enabling support for more spi interfaces
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>