mtd: spi-nor: set maximum frequency for SFDP reads at 50MHz

According to JESD216D:

  SFDP compliant devices must support 50MHz operation for the Read SFDP
  command (instruction 5Ah). Devices may support a wider frequency
  range, but a controller can always run SFDP cycles at 50MHz or less
  and get valid results.

Tell spi-mem to run SFDP commands at a maximum of 50MHz.

Signed-off-by: Pratyush Yadav <p.yadav@ti.com>