[sw, dif_uart] Remove null check for base_addr.base
[sw, dif_uart] Remove null check for base_addr.base

The intention of the memio_region_t is to be an opaque
container. Having this check in place makes the build fail with
c_args: ['-DMOCK_MMIO'].

Signed-off-by: Silvestrs Timofejevs <5a6466a3751cccdb6ee413d1e87698ed8baaba81@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan