soc: rockchip: rk3588: add SYS_GRF SOC_CON6 register offset

Add the RK3588_SYSGRF_SOC_CON6 register offset to the RK3588 GRF
header. This register contains the I2S MCLK output to IO gate bits,
needed by the clock driver.

Signed-off-by: Daniele Briguglio <hello@superkali.me>
Reviewed-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>
Link: https://patch.msgid.link/20260419-rk3588-mclk-gate-grf-v4-4-513a42dd1dcc@superkali.me
Signed-off-by: Heiko Stuebner <heiko@sntech.de>