mtd: spi-nor: Support TB selection using SR bit 6

mtd: spi-nor: Support TB selection using SR bit 6

There are some flashes to use bit 6 of status register for Top/Bottom (TB).
Use top/bottom bit variable instead of fixed value and support this case.

Set the Top/Bottom (TB) mask based on SPI_NOR_TB_SR_BIT6 flash_info flag.
We can't use a bigger granularity, for example to set TB_BIT6 per
manufacturer using a SNOR_F flag. The manufacturers don't have a common
rule in regards to the TB bit:

Winbond : Use the 6th bit from 32MB capacity
W25Q20EW, W25Q50BW, W25Q128V - TB(5)
W25Q256JV, W25M512JV - TB(6)

GigaDevice : Use the 6th bit from 32MB capacity
GD25Q16C, GD25Q32C, GD25LQ32D, GD25Q64C, GD25Q128 - TB(5)
GD25Q256 - TB(6)

Micron/STM : Keep to use 5th bit
M25PX64, N25Q128A, N25Q512A, MT25QL512ABB, MT25QL02GCBB - TB(5)

Spansion : Use the 6th bit from 16MB capacity
S25FL116K, S25FL132K, S25FL165K - TB(5)
S25FL128L, S25FL256L - TB(6)

We can't make a correlation between TB and BP3 either, i.e. assume that if
BP3 is defined then TB will be at BIT(6). Micron breaks this rule.

Signed-off-by: Jungseung Lee <js07.lee@samsung.com>
[tudor.ambarus@microchip.com: describe the reason for setting a
new flash_info flag.]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>