pata/at91: use newly introduced SMC accessors

pata_at91 driver is broken since faee0cc3:
"make smc register base soc independent"
Fix it with newly introduced SMC accessors.

The overall action of removal of at91_sys_read/write will allow
to use the pata_at91 on a single zImage kernel.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: linux-ide@vger.kernel.org