ARM: imx: fsl_esdhc: fix usage of low 4 bits of sysctl register

The low four bits of the SYSCTL register are reserved on the USDHC
controller on i.MX6 and i.MX7 processors, but are used for clocking
operations on earlier models.

Guard against their usage by hiding the bit mask macros on those
processors.

These bits are used to prevent glitches when changing clocks on
i.MX35 et al. Use the RSTA bit instead for i.MX6 and i.MX7.

>From the i.MX6DQ RM:
	To prevent possible glitch on the card clock, clear the
	FRC_SDCLK_ON bit when changing clock divisor value(SDCLKFS
	or DVS in System Control Register) or setting RSTA bit.

Signed-off-by: Eric Nelson <eric@nelint.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>
Reviewed-by: Hector Palacios <hector.palacios@digi.com>