[ast] Add 'plusargs' to set Calibrated SYS clock to 70MHz
[ast] Add 'plusargs' to set Calibrated SYS clock to 70MHz

Signed-off-by: Jacob Levy <57317008739bda624200879251c2a07202f9dc7d@nuvoton.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan