drivers: serial: pl011: improve include list

<soc.h> was included because some CMSIS helpers (__DMB/__ISB) were
needed. In ARM SoCs, inclusion of CMSIS headers depends mainly on how
HALs decide to do it, being usually an inefficient and fragile include
chain. Note that on ARM64 we're in a better position, as those are
defined in-tree.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>