mtd: atmel-nand: use pmecc_readl(b)/pmecc_writel to access the pmecc register

We defined the macro pmecc_readl(b)/pmecc_writel for pmecc register access.
But in the driver we also use the readl(b)/writel.
To keep consistent, this patch make all use pmecc_readl(b)/pmecc_writel.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>