module: Introduce sc_pll driver module
The PLLs in both the Morello and N1SDP platforms are Silicon Creations-provided IPs and are present as product-specific modules within the each product.
Generalise the Silicon Creations PLL driver by making it a common SCP driver module. The private PLL module(s) and references to them will be removed from the N1SDP and Morello products in the imminent commits.