firmware: raspberrypi: print time using time64_t

The firmware timestamp is an unsigned 32-bit value, but we copy it into
a signed 32-bit variable, so we can theoretically get an overflow in
the calculation when the timestamp is between 2038 and 2106.

This changes the temporary variable to time64_t and changes the deprecated
time_to_tm() over to time64_to_tm() accordingly.

There is still an overflow in y2106, but that is a limitation of the
firmware interface, not a kernel problem.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>