drivers: memc: rename flexspi-hyperram driver to flexspi-s27ks0641

Rename flexspi-hyperram driver to flexspi-s27ks0641, and update
function names. This driver is only capable of supporting the
s27ks0641 HyperRAM chip, as the lookup table given in this driver
is specific to the s27ks0641.

Rename the flexspi-hyperram binding to reflect this, to
prevent confusion from users.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>