Reland "Kconfig: Bring HEAP_SIZE to a common, large value"

This reverts commit acbc491237.

Reason for revert: CB:79525 fixes the issue that led to the revert
by not maintaining the heap in the SMM-stored copy of ramstage at all.

Change-Id: I3c8ef785486d275c9341859d34fce12253bd2bb9
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80023
Reviewed-by: Sean Rhodes <sean@starlabs.systems>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/Kconfig

@@ -759,8 +759,7 @@ config RTC
 
 config HEAP_SIZE
 	hex
-	default 0x100000 if FLATTENED_DEVICE_TREE
-	default 0x4000
+	default 0x100000
 
 config STACK_SIZE
 	hex