Currently, only `pr_info!` (for the minimal sample) and
`pr_emerg!` (for the panic handler) are there.

Add the other levels as new macros, i.e. `pr_alert!`, `pr_crit!`,
`pr_err!`, `pr_warn!`, `pr_notice!` and `pr_debug!`.
Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
Co-developed-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Co-developed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Sergio Gonzalez Collado <sergio.collado@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>