serial: lantiq: Use readl/writel instead of ltq_r32/ltq_w32

Previous implementation uses platform-dependent functions
ltq_w32()/ltq_r32() to access registers. Those functions are not
available for other SoC which uses the same IP.
Change to OS provided readl()/writel() and readb()/writeb(), so
that different SoCs can use the same driver.

Signed-off-by: Songjun Wu <songjun.wu@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>