coreboot: Use generic base address

coreboot: Use generic base address

Since now the generic console_t structure holds the UART base address as
well, let's use that generic location for the coreboot memory console.
This removes the base member from the coreboot specific data structure,
but keeps the struct console_cbmc_t and its size member.

Change-Id: I7f1dffd41392ba3fe5c07090aea761a42313fb5b
Signed-off-by: Andre Przywara <andre.przywara@arm.com>