arm: mx6: ddr: fix cs0_end calculation

Current way of calculation CS0_END field for MMDCx_MDASP register
is problematic because in most cases the user is forced to define
cs_density in an unnatural way: as value - 2, instead of value.

This breaks the abstraction provided by struct mx6_ddr_sysinfo
because the user is forced to be aware of the way the calculation
is performed.

Refactor the calculation.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: NNikita Kiryanov <nikita@compulab.co.il>

arch/arm/cpu/armv7/mx6/ddr.c

@@ -309,9 +309,7 @@ void mx6_dram_cfg(const struct mx6_ddr_sysinfo *sysinfo,
 	twtr = ROUND(MAX(4 * clkper, 7500) / clkper, 1) - 1;
 	trcd = trp;
 	trtp = twtr;
-	cs0_end = (4 * sysinfo->cs_density <= 120) ?
-		   4 * sysinfo->cs_density + 7 :
-		   127;
+	cs0_end = 4 * sysinfo->cs_density - 1;
 
 	debug("density:%d Gb (%d Gb per chip)\n",
 	      sysinfo->cs_density, ddr3_cfg->density);