phy-mvebu-a3700-utmi: Use devm_platform_ioremap_resource() in mvebu_a3700_utmi_phy_probe()

Simplify this function implementation by using a known wrapper function.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>