dt-bindings: memory: mt7621: add syscon as compatible string

The syscon string was introduced because the mt7621 clock driver needs to
read some registers creating a regmap from the syscon. The bindings were
added before the clock driver was properly mainlined and at first the clock
driver was using ralink architecture dependent operations rt_memc_* defined
in 'arch/mips/include/asm/mach-ralink/ralink_regs.h'.

This string is already there on the memory controller node on mt7621.dtsi.

Add syscon as a constant string on the compatible property, now that memc
became a syscon. Update the example accordingly.

Fixes: 5278e4a1 ("dt-bindings: memory: add binding for Mediatek's MT7621 SDRAM memory controller")
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>