arch: *: prep_c: remove check for CONFIG_SOC_PREP_HOOK

soc_prep_hook() is always called from z_prep_c() which is implemented
as a C function. As such, there is no need to check for the associated
CONFIG_SOC_PREP_HOOK since the platform/hooks.h header will define hooks
as no-op function-like macros if their associated Kconfig isn't enabled.

Remove the Kconfig check from all arch implementations of z_prep_c() and
call soc_prep_hook() directly instead, to avoid duplicating the Kconfig
check already performed in platform/hooks.h

Signed-off-by: Mathieu Choplain <mathieu.choplain-ext@st.com>