mmc: mmc_of_parse: Enable 52 MHz support with "cap-mmc-highspeed"

"cap-mmc-highspeed" enables support for 26 MHz MMC, but there is no
additional flag to enable 52 MHz MMC. In Linux. "cap-mmc-highspeed"
is used for MMC HS at both 26MHz and 52MHz.

Use the same approach and enable MMC_CAP(MMC_HS_52) host capability
when "cap-mmc-highspeed" is found in the devicetree. In the event an
MMC card doesn't support 52 MHz, it will be clocked at a speed based
on its EXT CSD, even on 52 MHz host controllers

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Tested-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>