serial: max310x: use a separate regmap for each port

The driver currently does manual register manipulation in
multiple places to talk to a specific UART port.

In order to talk to a specific UART port over SPI, the bits U1
and U0 of the register address can be set, as explained in the
Command byte configuration section of the datasheet.

Make this more elegant by creating regmaps for each UART port
and setting the read_flag_mask and write_flag_mask
accordingly.

All communcations regarding global registers are done on UART
port 0, so replace the global regmap entirely with the port 0
regmap.

Also, remove the 0x1f masks from reg_writeable(), reg_volatile()
and reg_precious() methods, since setting the U1 and U0 bits of
the register address happens inside the regmap core now.

Reviewed-by: Andy Shevchenko <[email protected]>
Signed-off-by: Cosmin Tanislav <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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