ARM: OMAP2+: gpmc: Fix writing in gpmc_cs_set_memconf

ARM: OMAP2+: gpmc: Fix writing in gpmc_cs_set_memconf

Some GPMC_CONFIG7 register bits marked as "RESERVED", means they
shouldn't be overwritten. A typical approach to handle such bits called
"Read-Modify-Write". Writing procedure used in gpmc_cs_set_memconf()
utilizes RMW technique, but implemented incorrectly. Due to obvious typo
in code read register value is being rewritten by another value, which
leads to loss of read RESERVED bits. This patch fixes this.

While at it, replace magic numbers with named constants to improve code
readability.

Signed-off-by: Semen Protsenko <semen.protsenko@globallogic.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>