soc: imx: gpc: remove unnecessary readable_reg callback

It is not really necessary to provide the current .readable_reg
implementation as we know what we're doing in our driver
and the regmap core has already done the partial check for
available maximum regs.

Acked-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>