soc/cdns/sample_controller32: align `CONFIG_PRIVILEGED_STACK_SIZE`

This commit aligns the value of `CONFIG_PRIVILEGED_STACK_SIZE` to the
alignment specified by the value of `XCHAL_MPU_ALIGN`, which for
`sample_controller32` is equal to 4096.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>