watchdog: mpcore_wdt: Rename dev to pdev for pointing to struct platform_device

Pointer to struct platform_device is named as dev, which makes it confusing when
we write statements like dev->dev to access struct device within it.

This patch renames such names to pdev.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>