[ARM] rtc-pcf8583: Final fixes for this RTC on RiscPC

Replace the I2C bus address, as per drivers/acorn/char/pcf8583.c.

Also, since this driver also contains Acorn RiscPC specific code
for obtaining the current year from the SRAM (and updating the
platform specific checksum when writing new data back) this is
NOT a platform independent driver.

Document it as such, and update the dependencies to reflect this
fact.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>