ARM: dts: dra7xx: Update spi-max-frequency for QSPI

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

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>