Support EWRAM wait state configuration through GBA register 4000800h

[HorstBaerbel]

The GBA (Regular + SP, NOT GBA Micro or DS) support EWRAM wait state control through "undocumented" register 4000800h (see GBATek). Support would be nice. Atm mGBA reports:

[GAME ERROR] GBA I/O: Write to unused I/O register: 800
[GAME ERROR] GBA I/O: Write to unused I/O register: 802

Code:

#define REG_WAITEWRAM (*(volatile uint32_t *)(REG_BASE + 0x0800))
// set undocumented register for EWRAM waitstates
// sets EWRAM wait states to 2/2/4 vs. 3/3/6 (default 0x0D000020)
REG_WAITEWRAM = 0x0E000020;

[endrift]

I haven't added support for remapping/disabling EWRAM via the low bits yet, but this takes care of the easier stuff.