soc: riscv: priv: call __reset during boot

Call __reset instead of directly calling __initialize from the common
RISC-V privileged SoC vectors __start. This allows injection of SoC
specific reset code just after setting up the machine trap vector.

RISC-V privilege SoCs without the need for custom reset code can set
CONFIG_INCLUDE_RESET_VECTOR=y to include a __reset stub which simply
calls __initialize.

Fixes: #38396

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>