serial: fsl_lpuart: DMA support for 32-bit variant

Add DMA support for 32-bit variant of the LPUART, such as LS1021A.
Signed-off-by: Tomonori Sakita <tomonori.sakita@sord.co.jp>
Signed-off-by: Atsushi Nemoto <atsushi.nemoto@sord.co.jp>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>