mmc: sdhci-esdhc-imx: optimize clock loopback selection with dummy pad support

For legacy platforms without dummy pad:
When clock <= 100MHz: Set ESDHC_MIX_CTRL_FBCLK_SEL to 0 (external clock
pad loopback) for better bus clock proximity.
When clock > 100MHz: Set ESDHC_MIX_CTRL_FBCLK_SEL to 1 (internal clock
loopback) to avoid signal reflection noise at high frequency.

For i.MX94/95 with dummy pad support:
Keep ESDHC_MIX_CTRL_FBCLK_SEL at 0 for all speed mode. Hardware
automatically substitutes clock pad loopback with dummy pad loopback
when available, eliminating signal reflections while preserving better
bus clock proximity.

Signed-off-by: Luke Wang <ziniu.wang_1@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20250521025502.112030-2-ziniu.wang_1@nxp.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

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