serial: sh: Add DEBUG_UART support

Add support for debug output very early during boot using the DEBUG_UART
mechanism. This uses a static fixed UART port configuration selected via
Kconfig options and dedicated print functions from debug_uart.h. This is
useful e.g. when debugging problems so early during boot, that not even
the DM is initialized at that point, and thus DM_SERIAL is not available
either.

This functionality is disabled by default. To activate it, define the
following Kconfig options and select SCIF type using CFG_SCI/CFG_SCIF_A/
CFG_HSCIF/<nothing for regular SCIF>:

CONFIG_DEBUG_UART=y
CONFIG_DEBUG_UART_SCIF=y
CONFIG_DEBUG_UART_BASE=0xe6540000
CONFIG_DEBUG_UART_CLOCK=24000000

The later two options define the SCIF physical base address and SCIF
input clock in Hz. Optionally, to validate DEBUG_UART works, enable
the following as well to get early serial output message by default:

CONFIG_DEBUG_UART_ANNOUNCE=y

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>

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