mtd: rawnand: meson: support for older SoCs up to Meson8

This adds support for the following SoCs to the meson-nand driver:
- Meson8 (assuming Meson8m2 uses the same IP block revision)
- Meson8b
- GXBB

The clock setup is the only difference between GXL and the older SoCs.
Compared to GXL and AXG these older SoCs:
- have a dedicated NAND clock instead of one that's shared with
  sd_emmc_c
- use the same ECC capabilities as GXL (up to 60-bits ECC)

The "amlogic,mmc-syscon" property is not required on these older SoCs
because the NAND clock is not shared with the sd_emmc_c controller. The
syscon from that property is used to switch the clock output between the
NAND controller and the sd_emmc_c controller on the newer SoCs.

The "rx" and "tx" clocks also don't exist on the older SoCs which also
means that the phase cannot be controlled there. Obtain these clocks
using devm_clk_get_optional() which will return NULL if they were not
passed via device-tree. None of the "consumers" of the "rx" and "tx"
clocks (within the meson-nand driver) have to be adjusted because the
common clock framework is NULL-safe (meaning NULL-clocks can be passed,
clk_prepare_enable() and clk_disable_unprepare() are no-ops in that
case).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>