mfd: intel-m10-bmc: Fix the register access range

This patch fixes the max register address of MAX 10 BMC. The range
0x20000000 ~ 0x200000fc are for control registers of the QSPI flash
controller, which are not accessible to host.

Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>