fsl_esdhc: Remove cache snooping for i.MX

The cache snooping feature of Freescale's eSDHC IP is not available on i.MX, so
disable it globally for this architecture. This avoids setting no_snoop for all
i.MX boards, and it prevents setting a reserved bit of a reserved register if
fsl_esdhc_mmc_init() is used on i.MX, like in
arch/arm/cpu/armv7/imx-common/cpu.c/cpu_mmc_init().

Since no_snoop was only used on i.MX, get rid of it BTW.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Kim Phillips <kim.phillips@freescale.com>