drivers: ram_console: add option to link console buffer to given section

Currently, the ram_console buffer is defined as a global var, its
address is determined during the building and may be changed when
code update. This is not a problem if the ram_console is just used
in debug purpose.

While in the heterogeneous SoCs, there can be multiple MPU Cores
and several MCU cores, it can run multiple OS/bare-metal instances
on these cores, but the UART ports may be not enough, so the
ram_console can be leveraged. To make it easy to use, it's better
make the console buffer fixed and predefined.

This patch adds a option to link the console buffer to a given
section, through the "zephyr,memory-region" device tree node, then
the address can be known from the device tree node and easy to
check from other cores running Linux/U-Boot.

To use this option, the chosen property 'zephyr,ram-console' must
be added, the following is a example:

	chosen {
		zephyr,ram-console = &ram_console;
	};

	ram_console: memory@93d00000 {
		compatible = "zephyr,memory-region";
		reg = <0x93d00000 DT_SIZE_K(4)>;
		zephyr,memory-region = "RAM_CONSOLE";
	};

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>