The Synopsys DesignWare core supports 52 MHz MMC mode and 50 MHz SD
mode, so add the cap flags so they are used.

This works on socfpga.  The other user of this driver is Rockchip and
the datasheet I found for the RK30xx indicated it supported highspeed
modes as well.

The Linux kernel has DT properties, e.g. "cap-mmc-highspeed", but none
of the drivers in barebox support this at all.  They all specify flags
in their code.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>