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Allowing console base address to be set to NULL conveniently
allows console driver to ignore further invocations to console_putc()
and console_getc(). This patch adds `console_uninit()` API to the
console driver which sets console base address as NULL. The BL images can
invoke this API to finish the use of console and ignore any further
invocations to print to the console.

Change-Id: I00a1762b3e0b7c55f2be2f9c4c9bee3967189dde