spi: spi-nxp-fspi: don't depend on a specific node name erratum workaround

spi: spi-nxp-fspi: don't depend on a specific node name erratum workaround

In commit 7e71b85473f8 ("arm64: dts: ls1028a: fix node name for the
sysclk") the sysclk node name was renamed and broke the erratum
workaround because it tries to fetch a device tree node by its name,
which is very fragile in general. We don't even need the sysclk node
because the only possible sysclk frequency input is 100MHz. In fact, the
erratum says it applies if SYS_PLL_RAT is 3, not that the platform clock
is 300 MHz. Make the workaround more reliable and just drop the unneeded
sysclk lookup.

For reference, the error during the bootup is the following:
[    4.898400] nxp-fspi 20c0000.spi: Errata cannot be executed. Read via IP bus may not work

Fixes: 82ce7d0e74b6 ("spi: spi-nxp-fspi: Implement errata workaround for LS1028A")
Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20211001212726.159437-1-michael@walle.cc
Signed-off-by: Mark Brown <broonie@kernel.org>