pinctrl: rockchip: rv1126: Fix support for IOMUX_L_SOURCE_PMU flag

GPIO0_C0-C4 iomux is set using PMUGRF_GPIO0C_IOMUX_L reg on RV1126. This
is indicated using the IOMUX_L_SOURCE_PMU flag. Fix reading current mux
by fully adopting the IOMUX_L_SOURCE_PMU related code in Linux kernel.

Based on Linux commit fd4ea48688c6 ("pinctrl: rockchip: Add RV1126
pinctrl support").

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>