mfd: as3722: Make FUSE7_REG readable

The FUSE7_REG register is not currently marked readable. This causes
as3722_sd0_is_low_voltage() to emit an error during boot, and assume
the range of the SD0 regulator:

as3722-regulator as3722-regulator: Reg 0xa7 read failed: -5

Fixes: d4807ad2 ("regmap: Check readable regs in _regmap_read")
  [exposed the bug, by checking for readability]
Fixes: 762a8ee8 ("regulator: as3722: detect SD0 low-voltage mode")
  [left out this register from the readable list]

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>