dts:sdmmc: slow down sdmmc frequence to 37.5MHz for SDK board

dts:sdmmc: slow down sdmmc frequence to 37.5MHz for SDK board

SDK board sdmmc can't run 50MHz, otherwise SD initialising will return error.
log as below. slow down sdmmc frequence to 37.5MHz can fix it.

<4>[  802.611901]  [0:    kworker/0:2:  644] rk_sdmmc: BOOT   The card is inserted.  ===!!!!!!==[mmc1]
<6>[  802.838728]  [0:  kworker/u16:0:    6] mmc_host mmc1: Bus speed (slot 0) = 50000000Hz (slot req 400000Hz, actual 3
96825HZ div = 63)
<4>[  802.838858]  [0:  kworker/u16:0:    6] rk_sdmmc: BOOT dw_mci_setup_bus: argue clk_mmc workaround out 800000Hz for
init[mmc1]
<6>[  802.990527]  [0:  kworker/u16:0:    6] mmc_host mmc1: Bus speed (slot 0) = 50000000Hz (slot req 50000000Hz, actual
50000000HZ div = 0)
<4>[  802.990671]  [0:  kworker/u16:0:    6] rk_sdmmc: BOOT dw_mci_setup_bus: argue clk_mmc workaround out normal clock
[mmc1]
<3>[  802.996461]  [0:  kworker/u16:0:    6] mmc1: error -84 whilst initialising SD card

Signed-off-by: hecanyang <hcy@rock-chips.com>