There are two problems in enabling DDR mode in this new driver:
1. The TDH bits in FLSHCR register should be set to 1. Otherwise, the TX DDR delay logic
won't be enabled. Since u-boot driver does not have DDR commands in LUT. So this won't
cause explicit problem.
2. When doing read/write/readid/erase operations, the MCR register is overwritten, the bits
like DDR_EN are cleared during these operations. When we using DDR mode QSPI boot, the TDH bit
is set to 1 by ROM. if the DDR_EN is cleared, there is no clk2x output for TX data shift.
So these operations will fail.
The explicit problem is users may get "SF: unrecognized JEDEC id bytes: ff, ff, ff" error
after using DDR mode QSPI boot on 6UL/ULL EVK boards.
Signed-off-by: Ye Li <firstname.lastname@example.org>