arch: arm64: smp: remove redundant soc.h include

<soc.h> has been traditionally been used as a proxy to HAL headers,
register definitions, etc. Nowadays, <soc.h> is anarchy. It serves a
different purpose depending on the SoC. In some cases it includes HALs,
in some others it works as a header sink/proxy (for no good reason), as
a register definition when there's no HAL... To make things worse, it is
being included in code that is, in theory, non-SoC specific.

This patch is part of a series intended to improve the situation by
removing <soc.h> usage when not needed, and by eventually removing it.

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