dt-bindings: serial: brcm,bcm6345-uart: convert to the json-schema

This helps validating DTS files.

Redundant generic descriptions (e.g. "The base address of the UART
register bank") were dropped and periph_clk was dropped from example as
not required.

Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20210829131927.13682-1-zajec5@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>