[sw/dif] Migrate UART DIF to _REG_OFFSET macros
[sw/dif] Migrate UART DIF to _REG_OFFSET macros

Since the reggen changes to generate `*_REG_OFFSET` preprocessor
definitions landed (lowrisc/opentitan#1118), we no longer need to fake
the UART base address to generate the register offsets. Instead the
generated header now provides new C preprocessor definitions for the
offsets.

This commit updates the UART DIF to stop faking the UART base address,
and instead uses the new preprocessor definitions.

Signed-off-by: Sam Elliott <32a4841301e2f444934d6e427a2764482fa7a43d@lowrisc.org>

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