pcm038: add driver for static ram

The pcm038 module (phyCORE-i.MX27) comes with a 512 KiB static RAM which
can be battery buffered. Add mtd_ram support and configure the chip select
line, to which the sram is attached.

Signed-off-by: Luotao Fu <l.fu@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>