serial: sh: Add HSCIF support for R-Car SoC

Provide the basic HSCIF support for R-Car SoC.

Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Hai Pham <hai.pham.ud@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
[Marek: Fill in HSSRR offset for Gen2 and SCBRR calculation for Gen2 and Gen3]
Reviewed-by: Simon Glass <sjg@chromium.org>