xen/arm: Rename all early printk macro

xen/arm: Rename all early printk macro

We are going to move the generation of the early printk macro into
Kconfig. This means all macro will be prefix with CONFIG_. We do that
ahead of the change.

We also take the opportunity to better name some variables, which are
used by only one driver and wouldn't make sens for other UART driver.
Thus,
    - EARLY_UART_REG_SHIFT became CONFIG_EARLY_UART_8250_REG_SHIFT
    - EARLY_PRINTK_VERSION_* became CONFIG_EARLY_UART_SCIF_VERSION_*

The other variables are change to have the prefix CONFIG_EARLY_UART_
when they change a parameter of the driver. So we have now:
    - CONFIG_EARLY_UART_BAUD_RATE
    - CONFIG_EARLY_UART_BASE_ADDRESS
    - CONFIG_EARLY_UART_INIT

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>