scsi: ufs: Change "<name>-max-microamp" to non-mandatory property

scsi: ufs: Change "<name>-max-microamp" to non-mandatory property

In dt-bindings for ufs, "<name>-max-microamp" property indicates current
limit and is mandatory if "<name>-fixed-regulator" is not defined on a
specified regulator.

However, in some platforms, regulators without "<name>-fixed-regulator"
property may not need to define their current limit because they may want
to define voltage range only for proper voltage switching in different
power modes, especially for vcc, vccq or vccq2.

Currently missing "<name>-max-microamp" property in device tree will lead
initialization to fail, thus such limitation shall be resolved to tolerate
this kind of regulators.

After resolving this, regulators without "<name>-max-microamp" property
will have undefined "max current" value, i.e., zero value in "max_uA" field
in struct ufs_vreg. Because we do bypass current switching operation (by
regulator_set_load) in case of undefined current limit, this patch shall be
safe.

Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Acked-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>