ARM: dts: dra7xx: Increase spi-max-frequency to 76.8MHz for QSPI

According to AM572x DM SPRS953A, QSPI maximum bus speed can be 76.8MHz.
Therefore, increase the spi-max-frequency value of QSPI node to 76.8MHz
for DRA74 and DRA72 evm. This improves flash raw read speed by ~2MB/s.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>