ARM: lpc32xx: use __iomem pointers for MMIO

ARM is moving to stricter checks on readl/write functions,
so we need to use the correct types everywhere.

Cc: Roland Stigge <stigge@antcom.de>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>