mtd: fsmc_nand: use relaxed variants of io accessors

Use relaxed variants of readl/writel accessors. readl/writel io accessors use
explicit dsb instruction which causes stalls in the processor core resulting
several cycles of delay for each access

Use relaxed variants where ever possible. This also results in an improved
read/write performance.

Signed-off-by: Vipin Kumar <vipin.kumar@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>