configs: dra7xx: Update QSPI speed to 76.8MHz

Now that QSPI driver can support 76.8MHz, update the
CONFIG_SF_DEFAULT_SPEED to the same value.

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>