Per ast1030_v07.pdf, AST1030 SOC doesn't have SCU300, the pclk divider
selection is defined in SCU310[11:8].
Add a get_apb_freq function and a class init handler for ast1030.

Signed-off-by: Troy Lee <troy_lee@aspeedtech.com>
Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20220401083850.15266-7-jamin_lin@aspeedtech.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>