sparc: Update LEON serial drivers to use readl/writel macros

sparc: Update LEON serial drivers to use readl/writel macros

Update the LEON2/3 serial driver to make use of the readl and writel
macros as well as the WATCHDOG_RESET() macro.

Add readl/writel and friends to the asm/io.h file.

Introduce the gd->arch.uart variable to store register address.

Lastly, remove baudrate scaler macro variables from board config. It
is now calculated in the serial driver using the global data variable.

Signed-off-by: Francois Retief <fgretief@spaceteq.co.za>