bmc: move memory region size variable to method
bmc: move memory region size variable to method

Use an accessor method with hard-coded values instead of inlining all
the values.  This avoids the situation where the value is declared but
not used.

Signed-off-by: Patrick Venture <36c53fa43f47f3fd67cb2fc38031827244d18a1f@google.com>
Change-Id: Iad7f2c06d705522520254056b18ffd136fdc1906

openbmc/phosphor-ipmi-flash