[sw, dif] Improve UART initialisation routine
[sw, dif] Improve UART initialisation routine

This change allows to specify the behaviour of the UART initialisation
routine. It might be desirable to keep the peripheral configuration
state after the jump from boot_rom to flash.

Fixes #1505

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

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