arm64: dts: ti: k3: mmc: fix dtbs_check warnings

Now the dtbs_check produces below warnings
 sdhci@4f80000: clock-names:0: 'clk_ahb' was expected
 sdhci@4f80000: clock-names:1: 'clk_xin' was expected
 $nodename:0: 'sdhci@4f80000' does not match '^mmc(@.*)?$'

Fix above warnings by updating mmc DT definitions to follow
sdhci-am654.yaml bindings:
 - rename sdhci dt nodes to 'mmc@'
 - swap clk_xin/clk_ahb clocks, the clk_ahb clock expected to be defined
first

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Aswath Govindraju <a-govindraju@ti.com>
Link: https://lore.kernel.org/r/20210115193016.5581-1-grygorii.strashko@ti.com